Can I rig a home theater amp to run in wall speakers?

If the Sonance "speaker box" you refer to is their 260 x 3 amp, don't sell it short.  It will do fine with in-walls.

Regarding the 3 unused channels, you may have to use some 1-into-2 rca splitter cables on the Marantz pre-outs.

I currently have the pre-out on my Marantz going to the Sonance to power 3 pairs of in-walls.  Kitchen, patio, and garage.

The sonance box in question is a speaker distribution box, not an amp.  

Do do the RCA splitters have any major negative effect?

" His new house has 8 in wall speakers distributed throughout the house."

8 mono speakers, 4 pair stereo. or 8 pair stereo? 

"using the pre-outs from the Marantz"

Stereo or A/V, and if A/V, main preout or zone 2 preout?

Any way to control volume of individual rooms?

"Do the RCA splitters have any major negative effect?"

I would be more concerned about the speaker distribution box. Some have as much as a 50% power loss. Do you know the model number of this box?

tls49,  thanks for the help!

he has 1 pair of outdoor speakers, 1 pair in living room, 1 pair in family room, a single speaker in kitchen, and a single speaker in a hallway (which is a low priority).

On the Marantz HT Receiver, he is using the main speaker outputs for his home theater, and then the stereo pre-outs to the Rotel amp.  Is there a better way to feed the Rotel here?  What is the A/V out?

no way to control volume in each zone

At this point, we are shooting for a minimally functional system!  I advised him that a great way to do this properly would be to purchase a Sonos Connect Amp to each pair of speakers for excellent control, but that is an expense he does not want to incur at the moment.  So, to use what he has will have to do for now.  Is there a way to disable the speakers on the Marantz HT Receiver while the pre-outs send signal to the other speakers?