How much difference between balanced XLR and RCA

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!

What's differential amp?

do you know if Proceed AMP5 or Bryston ST/SST Amps are differnetial?

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.
Enjoy!

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.