How much difference between balanced XLR and RCA

Balanced(XLR) vs RCA interconnects.

Does it make much of difference in sound quality?

I'm getting power amp with balanced in, and thinking about between Meridian 561 and Anthem avm20 pre/pro.

Meridain 561 looks like a great deal these days, but it lacks the balanced out...

Sort of feeling quilty of getting a pre/pro without balanced out while my amp will have the balanced in...

Virtually everyone agrees that XLR works best for long runs of cable (my personal preference is for XLR end to end, regardless of length -- but my DAC's are only set up for balanced in and I use single ended out and they sound very fine). If your amp is a differential amp, I'd suggest sticking with the balanced. Most amps aren't (I believe). So, I'd suggest going with the preamp that makes you happy, which presumably sounds great, regardless of RCA or XLR. Think of it this way: Almost all turntables are single ended out and many audiophiles love them!
Ozfly pretty much has great advice. I have heard RCA's sound just as good as XLR in any given set up. One place I hear a nice improvement is in a longer run of cable from my pre amp to amp. I found balanced did 3 things better in my set up. It lowered the niose floor adding to dynamics, increased the bass response and gave a wider soundstage.
What's differential amp?

do you know if Proceed AMP5 or Bryston ST/SST Amps are differnetial?
Check with your manufacturer, some that I know of say that their component is designed to run either balanced or singled ended better than the other, but offer both ways for connection. Musical Fidelity for one beileves that their components sound best with singled ended, and thus, don't offer balanced outputs, or inputs.
Also depends if your living environment has alot of RF noise. Balanced filters all of that out. If you live out in the country, it doesn't matter. If you live in the heart of Manhattan, you may see some benefit.
I have heared many times that RCA out of a CD Player sounds much better than XLR. I think you should try yours out and see if it does or not.
Note: I use all XLR
Most important is the signal fed into any cable, balanced or RCA. Even the 'best' balanced cable can't improve a bad signal. The Meridian is a very nice piece of equipment, so if that sounds better to your ears, forget about the type of connections and enjoy the music.
Eandylee, a differential amp is one where each of the signals is run through it's own amplification stage: Right hot, right neutral, left hot, left neutral. This requires a very careful matching of output devices (e.g., FET's). That's my understanding but I'm not an engineer so I may have misinterpreted something along the way. In any event, they are best served by XLR cables. My phono preamp will not accept an RCA and I had to go through some very interesting cabling mods to get everything to work right.