Don't worry about splitting. You're just splitting a signal...nothing carrying any power (the power comes from the sub's amp and your shaker amp), so there's not much to worry about.
Seperate sub amp? You already said your sub is 100 Watts...which I assume means it's powered? In that case you're just sending it a signal (via a single subwoofer cable). It provides it's own power...absolutely no need (or way) to "amp" it seperately. Correct me if I'm misunderstanding you.
The way you're wiring (same as me, 3 in series per channel, 2 channels) you will have a 12 Ohm load (4+4+4) so you want your impedence switch set to 8 Ohms. This is important.
However, I found I had to run one string off the RIGHT channel for the A-speakers and the other off the RIGHT channel for the B-speakers...since the input is mono...I didn't getting anything out the left terminals. Somebody clue me in if I'm being dumb.
Anyway, welcome to the forums...they are just great aren't they? No place else on the web with this much quality info about this stuff. Highly addictive!
Shake shake shake! I can't wait to get home and try and figure out why there seems to be a difference between the last reciever I used and this new (er...old) one. It's bugging me that there seems to be a difference...there really shouldn't be. Both are Sonys. This shouldn't be this mysterious. Power is power in this case, right?
Last one did have an analog Bass knob on it...when turned too high seemed to muddy the shaking a bit...but when turned up just a bit, it kind of when from shake/vibrate to thud/kick. Hmm. Seriously, I might be just kind of imagining things at this point. I've already gotten so used to them. It's like staring at your projector for the 1000th time wondering "is my Green level set just right??"