As stated earlier:
Stereophonic consist of two separate channels. A right channel and a left channel to mimic what your two ears perceive.
A mono-Amplifier is an amplifier that amplifies one signal only. more of this latter.
The purpose of a mono amp for music reproduction purposes is:
1. Stereo amplifier (not dual mono stereo amps), amplifies a right and left stereo signals. They typically consisted of separate amplifier circuits but used the same power supply for both right and left channel circuitry.
The typical justifiable reason for mono block amplifiers is that the power supplies for mono blocks ("typically") are more robust than a single stereo amplifier's power supply.
However, there are stereo amplifiers that have correctly designed power supplies and therefore, you really won't hear a difference between that stereo amp and two identical (specs) mono amps.
There are amplifiers designed and constructed as dual mono amplifiers. They are basically two separate mono amps inside the same enclosure. They may share the power supply cable, but nothing else, and, if the power cord is designed correctly, you have no issues.
If you read the information pages from Nelson Pass on power supply design, you will see what it takes to correctly design power supplies.
The difference in cable length related to sound quality in my opinion is negligible. If the stereo amp is directly between the two speakers and mono amps are moved slightly closer to each speaker or behind them, we are only talking about a few feet difference. If one can hear difference in this case, then they have great ears and the cables are poorly designed in the first place. Add five more feet to correctly designed speaker cables, and one shouldn't hear a difference. If you are using balanced interconnect cables you definitely won't hear a difference. If you are using standard RCA cables that are well designed, add another five feet....you shouldn't hear a difference.
I wouldn't worry about cable lengths.
Its the power supplies that make the difference.
enjoy