Right /left channel gain imbalance

Very recently switched from integrated amp to separates(Bryston 4BSST/Audio Research SP16L pre). Using balanced interconnects in prior set-up, with current pre giving a single ended only option. Greater gain evident in right channel, and at a loss to figure this out. Up to this point, have not started any experimentation, other than to tamper with speaker placement. Would prefer advice from those wiser. Any thoughts/suggestions would be appreciated.
Weak left channel tube? Try switching IC's from pre-amp to amp, i.e. left to right, and see if the balance changes with the left speaker now having the greatest output. If it does, then try switching the pre-amp tubes left to right, etc, and see if the gain changes back to favoring the right channel. If it does, you have ID'd the likely problem, a weak left tube.
Newbee: Think I've found the culprit. Followed your suggestions so was able to rule out tubes or connectors. Last thing done was to switch the speaker cables. Lo, and behold, the gain shifted. This may well be an amp problem which I will have to take up with those concerned. Don't think this is a "home" fix. Do appreciate your response/input in that it put me on the trail. Thanks again.
Newbee: Update. Problem was an out of position switch on back of unit. All is well.