There are two sides to this debate, so there really is not a simple answer. Several good debates kicked up over the advantages or disadvantages of balanced interconnects shortly after the Model 990 was announced.
Balanced inputs were developed for use in studios, where the extreme levels of EMI and RFI "noise" tended to create a lot of grief for long interconnect runs. In those cases, there is a clear and undeniable advantage to balanced, which is why balanced are often recommended for long interconnect runs in home systems.
I'll link to a couple of good threads for you to read, but first here's a direct quote from forum member soundhound:
Be careful what you wish for.
Completely balanced circuits not only add to the complexity of a circuit by a factor of two, with the resulting addition of noise and distortion, but they also cancel out even order distortion components such as the 2nd and 4th harmonic, the very ones you do not want to be cancelled. This leaves only the odd order (3rd, 5th, 7th, 9th etc) distortion components laid bare, and these usually result in a sterile and hard sounding unit, even when the spec'd distortion is "vanishingly low".
All electronics generate distortion, however little, and the presence of the low order even harmonics (especially the 2nd harmonic) will cover up the presence of the higher order, harsher distortion components. Our ears are particularly sensitive to even vanishingly low levels of higher order/odd order distortion.
One of the major reasons tube amplifiers sound so natural is the fact that the distortion spectra is almost completely low order, with the 2nd harmonic being the most pronounced.
Plain old un-sexy unbalanced designs are the best sounding, and balanced circuits should only be used as a last resort to combat RFI/EMI picked up by extremely long cables (50' or more) that have to run through hostile environments.
Contrary to what marketing departments may have you believe, balanced circuits were developed for one purpose and one purpose only: to combat interference picked up by very long cables in professional installations. Their use actually involves a sonic penalty that is best avoided if at all possible.
That quote came from
here , and there was some good debate following it in that thread. There is also
this thread , and a few other discussions that have rattled around lately.