If your amp has a balanced input, I think you'll find that it will sound better if the preamp is balanced also. If you go with balanced, there is a standard for balanced operation (and there's no standard for single-ended, FWIW...). This standard is known as AES48 and defines how the grounding works and how the signal travels in the interconnect.
Most 'high end' audio preamp manufacturers seem to ignore this standard; I don't know if its thru ignorance or them knowing they can get away with it. But if the equipment supports the standard you will find that the interconnect cable (and how much you spent on it) will not affect the sound so much (if you've ever heard differences between single-ended cables you know what I'm talking about) and you'll have freedom from ground loops (which can increase distortion if they don't result in outright buzz).
When considering a balanced preamp, ask if it supports AES48. If you get 'what's that?' look elsewhere unless you don't mind flushing $$$$ down the loo.
