I feel your “pain”; I too am very sensitive to channel balance issues and one can really obsess over it. The first obvious question which you haven’t answered is have you always had this issue, or did you make an equipment change, including cabling, around the time that you started noticing the problem?.
**** I did swap the interconnect on the phono pre but couldn’t really tell if the imbalance moved or not. ****
This tells me that the issue is pretty subtle and probably with the room and not your gear; except perhaps with the subs. Otherwise, you should be able to clearly hear whether the imbalance jumped to the other side or not. Here’s what I would try and what worked for me (I too use a pair of subs, and I don’t have a balance control):
I assume you have your two subs situated L/R; one for each channel. First, turn off the subs and see (hear) if you still have the same issue. I would wager that the issue goes away. Bass frequencies and resulting room nodes are often the issue; it was in my case. Even if your room is symmetrical, if one side has a load bearing wall and the other does not, or has a closet and the other does not, each side will reinforce or absorb bass frequencies differently. The effect of this can impact our perception of the midrange. Then, swap the subs. It is possible that one is producing slightly more output than the other, or they are not exactly the same at the xover frequency settings. If nothing changes, then you know that the subs are not the problem and it is the room. I was able to, if not entirely fix the problem, greatly improve matters by slightly adjusting the output of one sub relative to the other.
Good luck.
Btw, don’t assume that because you have no ear wax that your hearing acuity is the same in both ears.