Bryston 3B ST with an AV Receiver?

I did this exact thing. Bryston 3BST with a Marantz SR880. It sounded great. You'll notice a big improvement to your 5.1 surround, untill you upgrade further.

I had the same problem. I have a Cyrus 2/PSX and wanted to keep it for 2 channel stereo. This is how I solved it. I purchased a Denon AVR 2800 for home theatre and ran the preouts for the front left and right to an input on the Cyrus 2 (Video in). I set the volume on the Cyrus once with the level utility in the Denon and marked the position. Now I listen to my Arcam CD player through the Cryus 2/PSX, as per normal, and when I want to watch movies I just switch the input on the Cryus to Video, set it to about eleven o'clock and away we go. Now I have the best of both worlds.

You could start with an AV reciever and then add the amp if it has pre-amp outputs. I used a Harmon Kardon AV Rec. for a while to drive an Aragon amp. But when I purchased a Pre-amp the sound improved drastically. Most mass-market recievers will compromise the sound and certainly will not allow the Bryston to perform to its limits. I would suggest buying a decent preamp with a surround processor loop then you could add a lower priced processor.

You will receive very little benefit in your sound by going this way. You might as well just live with the 2-channel sound from your AV receiver until the time you can move up to a better quality pre-amp. The difference is HUGE. If possible, you should look for a good quality preamp/processor combo such as the Sony 9000ES series, Lexicon, B&W, Chiro, and the list goes on. I own Bryston amps and they are so good and so clean that anything less than a dedicated preamp will not let them shine.