Would this amp/pre amp paring work?

Not to knock the receiver because it does a decent job at HT on a small scale; but a 2 ch. pre, and any decent amp will get you further on up the road. Maybe a McMcormace ADL or similar. It has the HT pass-thru. Then say a DNA 05---a '1', will do much better and still use the rec. for the rears. Both these can be had for say 12/used,as a pair. A decent tube number, capable of HT would cost that much alone. So it depends on how much we want to spend.

Your idea will work great! You should try quite hard to match the front 3 channels. Buying vintage may help you get 3 monoblocks so you don't have to waste an extra channel. The other solution that I would highly recommend is: Get 2 stereo amps that are bridgeable (ie dynaco ST-70) and use 1 for the right and left and the other bridged mono for the center. The center channel is the most demanding anyway and should have the extra power.
However, for the price of a pair of ST-70's you may end up in the price range of some nice solid state 5 channel amps like an Anthem?

Thanks for the feedback. I've got some research ahead of me.