I recently bought a Pinball Pro kit for my DE Star Wars. Some notes on my experience:
Installation was relatively simple, although I thought the instructions could have been clearer — they didn't seem to match the exact kit that I ordered, but I figured it out.
The speakers sound great.. amazing amount of low-end. I'm impressed with the hardware. Kudos to you if you can leave these speakers at their default levels! They literally shake the whole house.
Alas.. it was too much bass for me personally. I live in an old multi-floor brownstone and don't want my neighbors complaining to the landlord. Plus, I felt that the bass was overpowering to the rest of the game audio.
You get a volume adjuster in the kit that installs in the back box, but it's purpose is to reduce the volume of the backbox speakers only. The machine's standard volume control adjusts both the subwoofer and the back box speakers. Unfortunately, with this setup there is no way to reduce the bass level but keep the back box high. Pinball Pro offered to send me an inline resistor, but I wanted something with adjustability.
I ended up installing a 2nd L-Pad (50W 8ohm) next to the sub-woofer so I can now adjust the back box and subwoofer levels independently. Works great! I had to dial the bass level dial down to about 20% to get it where I like it.
Would be great to see Pinball Pro offer a sub-woofer L-Pad as an option when ordering. Or better yet, incorporate it into the base kit.
One downside of ordering a high-fidelity speaker kit for an older pinball game.. it makes it obvious how low-quality the audio data really is. You're not going to hear clearer treble because it's just not there in the system. You'll definitely get improved bass though.