Quoted from PinWoofer:

Wow - what a great compliment! Here's something from our blog that's relevant to the audio discussion over the past day:
Below is the account of what we did here at PinWoofer to our Jersey Jack Pinball (JJP) electronics in an attempt to improve the backbox sound field, and to address some of the constraints inherent in the platform.
First, the JJP backbox consists of a mid-range speaker and dome tweeter on each side. The speaker components themselves are well aligned to what one will find in a high fidelity speaker system having a separate tweeter/mid-range/(woofer) arrangement enclosed in a speaker cabinet. The JJP backbox speaker enclosure is at / near ear-level which differs from most other pinball platforms (honestly, it looks VERY cool). The JJP tweeters point directly at the player. But, here is the critical part - the mid-range backbox speakers point vertically and the sound is reflected off of internal wood panels, angled towards the player position. This is a unique speaker implementation in pinball. So, does it matter? We think it does because the mid-range is reflected towards the listener as opposed to facing the player.
Imagine your home theater system in your viewing room and your listening perspective being from an adjacent room or hallway. Ask yourself how would it sound. The reflections of otherwise clean audio will be altered by walls, floors, ceiling and doorways. Perhaps the result can be described as an "echoing" effect as mid-range sound energy is absorbed as it propagates and reflects towards the listener perspective. This of course is not ideal and this scenario may be a slight exaggeration of the backbox experience of a JJP title compared to a Stern title for example, but you get the point. The reflected mid-range sounds emanating from the backbox change as energy is lost due to the reflection, and the listener receives something that has been altered by the environment, easily described as a loss of mid-range fidelity. So, the conclusion is that the resulting audio from the listeners perspective is not at its full potential. (JJP uses good backbox mid-range speakers and if they were facing the player, they would sound better.) Skilled listeners hear the effects of this placement and readily understand the problem; casual listeners have simply described the sound as "tinny".
JJP has taken the approach of adding a sound bar on top of the CE backbox and while this is a potential means to partially "fix" the platform issues discussed above (that is, making up for reflected mid-range speakers), to do this yourself on a SE/LE, you now have additional wiring from the cabinet and additional power cables cluttering your area. You need a solution to affix the sound bar so it does not vibrate and fall onto the playfield glass. Add an external sub and these problems are compounded (a complex and messy solution). Also, and too important to not mention, are the lack of equalization and tone controls which is something that the PinWoofer GT amplifier provides as part of its rich feature set, but not available today in the menu settings of most / all JJP titles.
In an attempt to address / mitigate the above constraints, we set out to add back in some of the high bass and mid-range to temper the "tinny" character of the platform by making changes to our Expander Board(TM). In large part we feel that we were successful considering the constraints. Feedback has been positive for JJP titles and we are finding that we have something that has improved the backbox field and of course, there is no question the bass emanating from the cabinet sub is dramatically improved with the PinWoofer GT amplifier and speaker upgrade.
The network analyzer curve from our JJP Expander Board shows in part, how we have addressed backbox response through targeted filtering of the signal presented to the pre-amp. Even with observed high-end roll-off there is an abundance of high end treble.
It is unfortunate that you pay $8-12K for a world-class title (and JJP has some great ones) only to find yourself trying to select, add and to juggle external sound bars, subwoofers, audio splitters and cables, only to end up with a lackluster solution. You'll have no means to fine-tune the tone and levels from a central location. You are stuck with the defaults after all of that time, effort and expense. The cost of adding the parts and pieces for external components is comparable to the PinWoofer solution and our solution requires little effort to install and includes tech support. The PinWoofer solution is by far the easiest solution, delivering superior audio with perfect all-internal integration!
https://pinwoofer.com/blog/pinwoofer-jersey-jack-audio-overview/

Quoted from PinWoofer:

No worries jack8765 - you clearly read and understood our JJP blog post. (https://pinwoofer.com/blog/pinwoofer-jersey-jack-audio-overview/)
There are filters (including crossovers) in different parts of our system. However, it makes more sense to me to talk about what we did in terms of the audio spectrum so I attached a Bode Plot showing level versus frequency. What you see in the plot is the spectrum as we modify it, prior to feeding it into the PinWoofer pre-amp. What you might commonly think of in terms of a crossover point between the cabinet and backbox is somewhere between box A and box B, internally, because our cabinet and backbox amplifier stages are separate with individual level controls. Here are the design goals we used for JJP:
(Green Line - pre-amplifier input reference)
Goals:
Box A - increase bass to the cabinet and backbox stages,
Box B - increase mid-range to the backbox stage,
Box C - attenuate treble to the backbox stage.
So to answer your question, yes, we send more mids and high bass to the backbox. We also trail off the treble and while you might think box C is dramatic, it's not. There is an abundance of high end. So much so that guys usually report back with middle adjust of the treble control if not less. This is owing to the forward facing tweeters that are at ear level. When you add amplification and dynamic range it is necessary to attenuate the treble a bit in this case, but the result is a brilliant high-end.
While we do not claim to have solved the topology problem (the reflection of the mid-range drivers), this was our attempt to address / mitigate the issue as much as possible. We found the "tinny" effect to result from the C box (i.e. tweeters stronger than the mid speakers) so we biased the expander board to ring up the frequencies below that band, and attenuate the frequencies within it. We think it helped.
Sound Bars. The CE Sound Bar has an integrated mounting solution but adding one later is going to be a pain to secure and you'll have the wiring. Your results will also depend on the sound bar you select. If you do, be careful not to select a mono bar since you'll end up mashing stereo from the stock backbox with mono. Other concerns if its a 3-channel bar. A good stereo sound bar is not cheap and many units have small speakers (2" - 3") which don't really improve the mids and high-bass. When we tried it the sound field quality was not dramatically improved. Some sound bars have larger speakers but they scale up in cost, weight and size rapidly.
Adding external components are work to get right and may end up costing more than simply adding the PinWoofer system. It's debatable that you'll get a equivalent or better result and it certainly won't be integrated.
Best wishes regardless of what you choose to do; we're here to provide and stellar option that drops in quickly and delivers great results.
Here is the GnR system we recommend:
https://pinwoofer.com/pinwoofer-gt-8in-jjp-super-kit/
Thanks for your time!
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Quoted from Eskaybee:

Plenty of wow moments in GNR and plenty of live and let die moments as well. But more importantly, the game is addicting as shit!
My personal experience, GNR came down to timing and the perfect storm and so thankful to be in the right spot and the right time. I was in the market for a new pin in September and put a deposit in on sterns next JP2 run. I really wanted a JJP pin but had no interest in GNR. I had two music pins at the time and was getting burnt out on music pins that play the same 10 songs for 30-45 seconds. I was dead set on not getting GNR, until they did what they did (those assholes! Haha). They did everything perfectly with this pin and it just wow’d me, this is a music pin!
Needless to say, I sold a pin and picked up both JP2 and GNRLE. GNR flippers felt very lackluster compared to my other sterns and JP2. The sterns are just snappier with more umph. Well, as many of you know, I spent nearly a week tweaking my GNR flippers - from power settings, to realigning, to trying many different types of rubbers. Well, I went to play some JP2 last night for the first time in awhile, the flippers felt like toothpicks - I was shocked. Turned the game off and went back to GNR. I’m sure I’ll give my other sterns and my other new JP2 some love again in due time, but GNR is a beast and plays so dam fun.