Paul McGowan said he had the same experience and that he couldn't explain it either.@tomcy6 The explanation is simple- interconnect cables contribute to the system sound if not properly controlled. A raw passive volume control puts a series resistance between the source and the amp, essentially raising its output impedance. For this reason, bass and overall impact are reduced and the effects of the interconnect cable are more audible.
When the volume control is intimately involved with an active circuit that has a low output impedance (as you see in many line sections) this effect is reduced.