I think your idea will work. It wouldn't be my first choice though. I would eliminate the old receiver from the set-up, and add a good power amp. Chances are the seperate amp will be much better than the old receiver is. Most, receivers don't have the greatest amp section. There are a few I have heard that are very good though. (I don't know if your old Yammy is one of them) I would put the old unit in my bedroom, or a 2nd system somewhere. If that really isnt an option, try your idea and see how you like it. You have nothing but some lost time if you don't like it. Good luck.
New home theater setup
I'm using some old and new equipment to set up a basement home theater. I have a great old Yamaha DSP-A2070 with lotsa power but prehistoric surround sound circuitry. Bought a new Yamaha Aventage RX-700 with nice surround circuitry but minimal power (90W?). I'm driving a pr. of B&W Matrix 3s, matching center channel and Paradigm Atoms for surround. Sub is a B&W ASW1000. I also have a 2nd speaker set-up out by the pool with some NHT Outdoor Ones. My thoughts are to pre-amp out the front channels to the A2070 and let the RX-700 handle the rest of the surrounds, as well as drive the NHTs. I would love input from folks who play with this stuff more than I do...