Yes - the audio going to the backbox speakers has all the bass filtered out, so I'd expect you to be pretty disappointed...

The cap forms a high pass filter, so that would be a 1-pole high pass (not unusual for 2-way speaker systems). My comment was with respect to fattdirk's DM (and all other WPC games, I believe).

I'd like to clarify this: you should clip to the speaker in the main cabinet - clipping to a speaker in the backbox (which is sometimes considered part of the "cabinet") will not work well because the signal to those speakers is high pass filtered (at least on the WPC-era Williams/Bally games I'm familiar with) so there won't be much signal there in the bass frequencies. The low-pass filter in the external sub would help some, and cranking up the sub gain would help some, but it would never sound as good as taking the signal from the main cabinet speaker (which is already low-pass filtered in the W/B-era games, exactly what you want with a sub).

No idea about other manufacturers/eras, but to be safe I'd always clip to the largest original speaker in the machine.