Yes, this is all good advice and I will use it in the continuing pursuit for a better setup of my "new" system. To answer the question of which EQ I'm using, I have one DBX 1531 set in mono for each speaker providing 31 band per channel control.
The advice given to pay attention to phase issues was high on my list especially since the addition of the pro type EQ where there is no polarity designation on the xLR to RCA adapter cables and the preamp and EQ are on different shelves. I have that issue in hand, though, but I've read that the equalizer itself can introduce phase problems. I don't understand why that would occur but it remains a question.
The suggestion that using a preamp with a mono/stereo selector would make troubleshooting easier but I'm pretty happy with my current preamp/ amp setup (Peachtree Audio) and would be reluctant to consider that significant a modification at this point. Besides, I've never even seen such a thing. As far as buying music recorded in mono is concerned, wouldn't it depend on how it was recorded? For example, one of the guitarists that I listen to, Adrian Legg, does much of his recordings in concert and it sounds good but with no sense of a centered stage.
Anyway, I'm taking a short vacation from this quest because there is a Christmas tree partly in front of my right speaker.