Most equipment that is balanced sounds best when run balanced - that is how it was voiced. Of course, in order to fully enjoy the benefits of a balanced preamp and amp, your source equipment should be balanced as well.
It's important to note that much (if not most) audiophile equipment that includes XLR inputs or outputs is not in fact balanced in the sense of featuring a fully differential balanced circuit -- the circuit is single-ended -- and offers XLR connections merely for the convenience of users owning balanced cables. It should also be pointed out that many manufacturers do not believe balanced equipment is desirable - Herve Delatraz of darTZeel, for example, is quite outspoken in his opposition to balanced circuits. I run a balanced Rowland preamp with balanced interconnects to a darTZeel amp, but the XLR inputs in my darTZeel (Version B) are connected to input transformers for purposes of noise reduction, not a balanced circuit. I also have a VAC Renaissance 70/70 amp that is single-ended and has only single-ended inputs - I use Neutrix XLR-to-RCA convertors with the balanced interconnect from my Rowland to feed it.
Your Parasound IS a balanced amp (and a great amp, by the way). Audio Research, Sonic Frontiers, BAT, Rowland and, I think, AtmaSphere are manufacturers who offer truly balanced preamps. If you like balanced tube preamps, I saw a Sonic Frontiers Line 3 for sale this morning in A-Gon's classifieds.
