I don't really use receivers, although I have some vintage HH Scott, Fisher, Marantz and etc laying around. I do a lot buying, selling, trading and find stuff cheap at an estate sales/thirft stores.
Is your sub passive or active(no amp in sub or amp in the sub)? If it's powered that makes things easier.
Amp wise, I usually have plenty around but for your case unless you want to start collecting more hi-fi components(since you have a good start anyways!). A 5 or 7 channel amp for simplicity is a good choice. Depending on the size of your room 5.1 might be plenty if you still want to go 7.1 you could use the Sony receiver to power 2 speakers. Possibly more depending on the receiver if it can pass through the a signal w/o the receiver doing any processing.
A lot of the 5 and 7 channel amps nowadays assume you are using a powered sub but once again you could use the Sony receiver to power that.
Also, Outlaw Audio is doing a combo package with the 975 Pre-amp processor and 7075 amp 7 channel for 948.00 US.
Depending on what speaker models you are using, it's hard recommend a specific amp for your setup but the rear and side speakers don't need much power. The front mains, sub and arguably the most important speaker in Home Theatre, the center channel need some decent power, but not a whole lot if the room is small to medium size.
Amp wise, you can look into the used market or have a stereo shop in your area that sell new and used, especially if they offer trade in options.