how to correct channel imbalance?

I seek the counsel of wiser tube mavens than myself. One channel of my setup is not quite as loud as the other. It is possible that speaker placement contributes, but I've gone as far as I can with that. But... with a tube amp, pre and phono (Mesa Baron, AR LS-15, EAR 834P), there are 13 twelve tubes involved in each channel, which would seem to suggest an opportunity for cumulative error or imbalance. Some are dual triodes (12ax7) that devote one side to each channel, the rest are EL34. All tubes are relatively young. Is is possible that some tubes are simply louder than others, and by switching or replacing tubes a better channel balance could be achieved? Is this a common problem, or is some other culprit more likely, e.g., a fading capacitor, a cartridge issue (I believe it is aligned well, using a db systems guide), or what? Must I give in get a pre with a balance control? I wonder whether the audiophile prejudice against balance and tone controls is misguided, but every equalizer I ever tried compromised the sound.
I bought a pre with a balance for this very reason (possibility).
I've been dealing with this exact same issue since I've been in my present room. In my case it is an asymetric room giving greater boundary reinforcement to one channel. When you have this issue with a room, balance controls may not entirely solve the problem as some frequencies may be reinforced more than others.

You can measure for any imbalance in your equipment by measuring voltage at speakers out on your amplifier. Play steady state tones at various frequencies and measure with the 2volt setting on DMM.
Well, it's pretty easy to determine if it is a speaker/placement/room issue. Simply swap the speaker cables at the amp end. If the problem stays with the original problem'd side, it's the speaker/placement/room, and not in the electronics. If the problem moves, then it is somewhere in the electronics. You can continue cable swapping unti you isolate it to one component. From there perform as many tube swaps between channels as possible to isolate it to as few tubes as possible.

I wonder whether the audiophile prejudice against balance and tone controls is misguided

Yes, I think so. I think most folks would be surprised to find that they have an acoustic imbalance in their listening rooms. I hope eventually we'll have 2-channel DSP gear (or we've moved to AV pre/pros) that solves these issues automatically. There are DSP controlled speaker systems that do it today.
thanks. I'll get a dmm and do more testing and switching.