For those who haven't seen the stock speaker, a picture is below. This is a mid-range driver that doesn't even have a whizzer (which is the inexpensive way to get some treble compared to a dedicated tweeter). This is why voices sound muffled.

20160830_191006 (resized).jpg

I purchased a Kicker 40CS54 https://www.amazon.com/Kicker-40CS54-5-25-2-Way-Speakers/dp/B00BGRMFVI
because it has a sensitivity rating of 90db/W/m compared to the Pioneer which is rated at 86db/W/m. Assuming these numbers haven't been fudged (and that's a big IF), installing a speaker with 3dB higher sensitivity should be equivalent to doubling the amplifier's power output, which can be important in such a low fidelity installation.

The improvement in the treble is immediately noticeable. I had no idea there was a snare drum in the main theme! Paired with an existing Polk external subwoofer, this sound is as good as I can hope for.

Here is a picture of the Kicker speaker behind the grill:
20160830_191851 (resized).jpg

These are not the stock Stern speakers which are 4". The Stern 4" speaker has a better high-frequency response to my ears. Generally, the larger the cone, the better the low-frequency response and the worse the high frequency response.

It's not that these speakers are bad - I think these speakers were an inappropriate choice. Put another way, I don't think the engineer for these cones ever intended to provided any type of high frequency output.

I looked at that as well. I became a little suspicious because 93dB is really good - possibly too good to be true for that price. Some really expensive gear (such as from Infiniti or Polk) aim for high efficiency and don't get 93dB, or only achieve 93dB in their top-of-the-line models.

The other things that I could not determine is how far the tweeter pod protrudes. The spacer in MMr is .75", so you would need to make sure the tweeter pod protrudes no further than that. The Kicker is completely flush, guaranteeing that it will fit behind the grill.

There is also a Rockford Fosgate that should fit fine, but with a slightly lower sensitivity and slightly lower price.

Here is a summary of models tested and confirmed so far:
Kicker 41DSC54 with 90dB/W/m sensitivity https://smile.amazon.com/Kicker-41DSC54-5-25-2-Way-Speaker/dp/B00I2YX1KW/ref=sr_1_5

Pioneer TSG1345R with 86db/W/m sensitivity https://smile.amazon.com/Pioneer-TSG1345R-5-25-Inch-2-Way-Speakers/dp/B00NF5EQHK/ref=sr_1_1

Possibilities that may be worth trying:
Rockford Fosgate R1525X2 with 88dB/W/m sensitivity https://smile.amazon.com/Rockford-Fosgate-R1525X2-5-25-Inch-Coaxial/dp/B00BF6HX9E/ref=sr_1_2

robbo105 brought up an interesting point in another thread. Flipper Fidelity sells their speakers as components. So, if you want an 8 ohm speaker so that you don't have to change from parallel to series, here it is:
http://flipperfidelity.com/parts-and-diy/speakers/flipper-fidelity-ff4-4-2-way-speaker-484.html

If one buys two of the above speakers, all one would have to do is solder in the new speakers, or just cut off the existing wires and crimp on some female spade connectors and be done.

Personally, I don't mind the wiring change and like the wide variety of car speaker options, but this is a nice lower fuss option.

I'm interested in how you wired the amplifier to switch on with the pinball machine.