I believe you were correct in your first post that synth music can go lower - or with known levels. A friend of mine was a huge car stereo nut and could play so loud so deep that you had to wear headphones while in the car to block out the sound as it could possibly intantly deafen you.

Trance/hip hop/rap kind of stuff can have very deep (syths cover the entire range+) and bass is the name of the game for dance clubs for people to "follow the bass line" which is why you see the kinds of speakers in nightclubs. Big impact bass, big horns. The best of which I have heard in a nightclub was a big set of Tannoy speakers with two 15 or 18 inch drivers and dual concentric. I don't remember the model number as a bunch of Koreans were feeding me Soju all night. Interestingly the brand of Soju was called Jinro which is the baby Ongaku amp.

I believe you can look on forums to find the bass head albums with synth under 20hz. This is where bigger speakers to me always tend to win out. You can't get away from it. For big deep bass at a macro-dynamic level at loud volumes (which is the point) that can hit you in the chest - those big Tannoy speakers are quite awesome. Panels? We don't need no stinking panels. Of course those big speakers don't fit most domestic living spaces so you are kind of forced to go smaller.

I have a couple of Kraftwerk LP's - maybe one of them will go down under 20hz - have not listened to them yet.