You're perfectly welcome to go this route, but before you part with the cash, it might be worth just trying to play with your current set-up. On my system I find that the musical enjoyment is vastly improved using the Extended Stereo setting which feeds the stereo signal through all available speakers, although I do alter the fader in favour of the front speakers about half way. Crucially, this isn't Dolby ProLogic.
i was genuinely surprised to hear results that were considerably better than Pure Direct, which I had previously assumed would be the best. I don't know what the equivalent setting would be on your Yammy amp.
The reason I find this interesting is that I accept that playing a stereo signal through two speakers powered by an AV receiver may not be quite so 'hi-fi' as a two channel amp through two speakers, but if you have a multi-channel set-up, why not play to its strengths and do something that a stereo amp is not capable of.
The 3D effect I get with my sound playing CD's is just wonderful, and I couldn't achieve this with a stereo amp.