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.