Granted, the 6500 doesn't have the uber-matrix, but it does a right fine job with what it's got. The QR series receivers strongest feature for me was always the flexibility. You can run two rooms of stereo as well as quad in the main room by just flipping the switches. If power is an issue (and 40 watts per channel is great, but 600 is better ;-} ), you can hook an outboard amp to the pre-out/main loop, get kick ass power to infinity or whatever your budget allows, and run three rooms stereo. For whatever reason, all the later Sansui quad units dropped the separate speaker selectors for front AND rear, and got rid of the pre-out/amp in loop. I miss those features. And you've already mentioned (or somebody did anyways) they are some classy looking beasts.
If you haven't got a lot of old quad source to play with, the QR is an excellent front end for an SACD player. I've got an old Carver cube with one good channel that I use for the center channel and run a big amp for the front channels, so don't need a sub. You end up with pure five channel, and still have the quad synth capability to open up your stereo source material.
I ran a 4500 for years (basically the same machine with less power output), "upgraded" to a QRX-9001 (which is now on the bench with typical QRX issues) and am currently running a QRX-6001 as the primary receiver. Admittedly, the fancy matrix does do some amazing things, but overall, compared to the QR, it sounds harsh to me. Somebody's sure to pipe in about me having a tin ear and all, and yes, there are maintenance issues with my QRX equipment, and yes, a lot of it is what you're used to, but there it is. Probably a lot like the old transistor vs tube argument, and don' get me started on the "Boston Sound" ...
The QRs aren't the maintenance hogs that the QRXs are getting to be. If you're not into all the bench work an unworked QRX would probably require to make it shine, you may have just made the best choice, especially at the price. Clean the controls, replace the lights if needed, plug it in, enjoy. You <may> benefit from a minor recap (just the biguns in the power path) as the old caps do lose oomph over time and can get a bit muddy on the deep bass.
Oh - the fuse lights in the dials and meters are getting kinda hard to find. Here's a link if you need them.
http://www.mcmelectronics.com/product/25-080
You'll need 8 and should plan to replace them all if you replace any. Probably best to order a couple extras for breakage as they can be a bit clumsy to get to. I let em burn overnite after replacing them - if they're all still working in the morning, chances are good they'll last for years.