It is true that with many transistor and push-pull tube amps, if they don't make enough power, their distortion is actually *higher*. With higher power amps that point is often between 5 and 10 watts.
A good reason to go balanced is the advantage of being able to run really long interconnects, so you can place the amps close to the speakers and avoid sonic degradation on account of the speaker cables.
The balanced line system was created to get rid of interconnect cable colorations. It works really well! However in order for that to happen, The preamp must support the balanced line standards (which have been in place for decades).
Most high end audio balanced preamps do not support the standard! As a result with such preamps you will encounter variable results as far as interconnects are concerned.
Here is the standard:
1) pins are: pin 1 ground, pin 2 and 3 are signal.
2) Ground is ignored- the signal occurs only between pin 2 and 3 (this is where most high end audio preamps have a problem- as soon as there are signal currents in the shield of the cable, the construction of the cable becomes critical).
3) the cable will be a twisted pair for the signal with a shield (tied to pin one only)
4) the output of the preamp should be capable of driving a low impedance load (2000 ohms or less) without loss of voltage, without increase in distortion and without loss of bass (this is the other big area where high end audio preamps have a problem, and also results in cable sensitivity).
Note: this does not mean that the output impedance of the preamp is this value, it means that it can *drive* this value. If there is a question, both the 1KHz output impedance and the 20Hz output impedance should be well below 300 ohms!
The actual standard is 600 ohms and you will have a lot of manufacturers of balanced products tell you that since the amps being driven have a much higher input impedance, that this does not matter. Such is incorrect if you want cable immunity! It is the higher impedance nature of single-ended preamps and amps that spawned the interconnect cable industry. Such is not needed for balanced as long as the standards are used.
Now some people want proof of this sort of thing- after all what I am saying here is that the interconnect should not have an audible quality in the system. So here is the proof. The first manufacturer of high end cables was FMI in Minneapolis, Minnesota. Robert Fulton, the proprietor, created the first cables in which in was claimed that they made an improvement when installed in a system. That was the late 1970s.
However in the late 1950s, the record labels were turning out recordings that are revered to this day (the better your system gets, the better these recordings sound). In many cases the microphone signal had to go over 200 feet to get to the input of the tape recorder- how did they do that without an exotic cable? The answer is the low impedance balanced line system.
So- if you want that same cable immunity that the recording and broadcast industry has enjoyed for the last 60 years, then your preamp should support the same standard. This takes the cable out of the system equation, and also its cost.
BTW, phono cartridges are a balanced source and should enjoy the same cable immunity if set up properly. If you have ever wondered why the phono system has that extra ground wire that no other single-ended source seems to have, that is why- its actually a balanced source that is being treated as if it is single-ended, but you have to do something with that ground...