Logic suggests that adding a complex preamp with many active components is likely to degrade sound by the distortion introduced in an otherwise pure signal passing from source to amp/speakers.
So what I'd like to hear on this issue from someone qualified is:
If a particular preamplifier can improve sound compared to eliminating it, what is it that the preamplifier adds or changes that makes the sound better?
And what is it that was wrong with the signal from source that required improvement?