What is the most overlooked consideration?

imho, people underestimate importance of clean power. I put in a new dedicated 20amp service for my "rig". I also spent 500 on a high end Furutech plug in the wall. These two actions made a Huge difference in sound.

That's two separate actions. The 20A home run to the panel may very well have made a difference for the better, depending on the state of your home wiring and the specifics of your system.

@mlapenta  Correct.  That Furutech combo is nuts.  I've not engaged in the dedicated 20A line, but do not doubt.  I've approached that part with a Puritan 156 conditioner, but you make a salient point attacking the source first.

Cumulative is the point here.  Improvement in all things has a cumulative affect in outcome.

This is a fascinating diversity of viewpoints.  Many valid ideas and perspectives have been raised so far.  It strikes me, though, that the fundamental question posed by the OP has not really been hit in the bullseye.  Perhaps the answer is so obvious that it goes without saying for most of us:  The first and most important consideration to be addressed every time we are going to buy anything related to our systems should be to decide from whom to buy it.  Or to state this as a first principle:  Pick your dealer first.

Synergy, through the entire audio chain. This will also include treating the room and having your ears professionally cleaned and detailed. 

Dual mono at a minimum