I have actually done this, but not all of it at the same time. This part on ce, that part another time, but there is no reason it can't all work togeth er.
Many people do not like new audio equipment, the tone controls suck, there is not much clipping headroom so that extra power is not there, to me most of it sounds like shit. And people might like how their old faithful amp so unds on their speakers. I have three amps and three pairs of speakers and t hey all sound different in different combinations, and I am half deaf !
This is true high fidelity surround there is no delay or effect. In fact wh en there is I do not consider it high fidelity because it is not putting ou t what is put in. With this, everything coming out of the speakers is exact ly what went into the amp. At the Asylum one said "You get one of those rec eivers and try all the modes, and then put it in direct mode and wonder why you bought it". I agree.
The circuit :
The amp must be of conventional design, not BTL or with one channel inverte d like some Carvers and who knows what else. If it is not conventional ther e may be a caveat in the manual about the use of speaker level fed amplifie d subwoofers, as they only work right with a conventional amp. However in o lder equipment before subwoofers were in widespread use there may be nothin g and you'll have to figure it out on your own.
The rears are in series out of phase, with the + to the same + as the front s. This mixes the sound in the room properly. There is no return. The resis tors combine the L & R into the center. The return from all but the rears t ie together and feed the sub.
Just off the top of my head, C 1 should be from 47 uF to maybe 100 uF, depe nding on the speakers, the sub and the desired crossover frequency.
I think 6 ohms should do for R 1 and R 2. That gives differential signal lo ad to the amp only 12 ohms in parallel, with the summed impedances of the r ears in series. It should not fry the amp though I would recommend one that can handle 4 ohms without burning the place down. This setup imposes a dif ferent type of load on the power amp which depends on how it is mixed down.
Two resistors from anywhere, a cap from anyone who sells speaker crossover components, the wire and speakers and you are in business.It should work fi ne with tube amps if you're into that. Myself, I prefer good solid state de sign. High damping factor. DC coupling. And the fact that unless it is a cl ass A I can just leave it on forever. You would never leave a tube amp runn ing 24/7/365. Also, this setup may decrease the stereo separation of a tube amp due to the low damping factor some have.
So, what y'all think of this contraption ? I can tell you it sounds pretty good, at least on most of what I listen to.