I usually go to around 48. Then you turn the vol control knob that you just installed down until the speakers are the right volume you want. This results in booming bass but normal volume speakers.
The vol knob will lower the vol of tgd backbox speakers. If this is not what's happening then you have it installed wrong.
What part of what I described is different for you?
Nothing - that's exactly how it's working, but if I turn up the machine volume control to get decent sound from the new backbox kit, the sub is knocking the walls down - which isn't what I wanted, or how the last kit sounded. I know WPC is different, I was just using the TZ setup as an example in terms of sound. I want to be able to turn the sub down, not turn the backbox speakers down which are already too quiet in comparison by a long, long way. The best tune in the game (Derezzed) sounds pretty bad now, it sounds thumping if you're familiar with it, but in reality there's not a huge amount at the lowest end. End result in gameplay is the booming from everything else totally drowns it out.
I'll probably put the stock speakers back in next week sometime, this just doesn't sound good.
All the other upgrades that went on today were nice. Flipper Fidelity LED backboard kit, I turned the animation to very slow colour fading, but even when switched to non animated it looks significantly better than the flourescent tube did. Shaker motor is pretty cool, and the I ripped out the EU model Pinled display and put a proper Babcock DMD in (along with the rest of the wiring and hardware you need). The animations look much better.