Channel imbalance with turntable playback

You need a Test Record to check channel orientations, phase and polarity. Look for Cardas Test LP or Hi-Hi News Test LP.
There is only one right position for all cables in your system from cartridge lead wires to your speaker cables. You have to check all of them using a test record!

You can swap whatever you want, but if you’re listening to a stereo source then your Left speaker is L channel and your right speaker is R channel, just like on the master. There is a simple test on any test record, the voice recorded on LP will tell you which channel it is originally, and if the voice appeared from the wrong channel then you must swap cables L R channels.

There is also phase and out of phase test, in polarity and out of polarity test.

Those TEST records are made for system adjustment.

If you want to know what's right and wrong USE A TEST RECORD!

So I’ve been having a slight channel imbalance for awhile now. Vocal tend to lean left and just overall left leaning gain.

Some records recorded this way

I swapped the L&R input on the phono preamp and it sounds way more balanced.

If you prefer to hear vocal on opposite side how do you know how it was recorded ? You can prefer what you're hearing on one particular record, but how about some other records ? 

You need a MONO record to check your system for imbalance. Mono signal is equal in both channels. Play MONO LP and try to detect imbalance. 

Also compare with digital source

even if I do find phase or polarity problems, how would I go about adjusting those?

Your cables have color code, equipment have L/R marks and color code, cartridge pins have color code too. If you made a mistake once then everything else is wrong, you must find where you made a mistake (check all cables in your system). 

Yes if you swap one cable, left goes to right and right goes to left, but if you switch two sets of cables. you are back to the proper side but using the opposite side of the amp or preamp. There are no rules for using the left side channel of an amp to run the right side signal to the right hand speaker.

Not sure what are you trying to explain to me. Are you aware of phasing, absolute polarity and channel orientation? We must listen music just like it was recorded, if you play music in the iPhone (it has two speaker now) you can rotate the iPhone 180 degree (for example), but only one position is right (true to the source) in stereo recording. If the OP prefer to listen his system virtually rotated 180 degree that’s not right!

There is a right and left speaker in the mastering studio when musicians recorded, left is always left and right is always right. Same on the cutting head. You must listen to the stereo sound exactly how it was recorded (picket up by the microphones in the studio). You can’t change position of the musicians, if you will do that by swapping L and R channel cable you will ruin everything. You can swap disconnected L and R speakers - it doesn't matter, but if you swap connected speaker then you must swap cable ends on the amp terminals too. 

I think it's absolutely crazy thread, sorry. 

-Use MONO record to check the imbalance.

-Make sure to buy TEST LP for stereo test (it will help you to understand what's going on).

-Use different stereo LPs to make sure what you're hearing.