It is most probably the phono preamp. There may be a balance control internally.
If you are fussy about balance and anybody with a system that images correctly will be you will be adjusting balance almost with every record or file. Fine balance is a preference issue plus no system puts out exactly the same level in both channels. If the mixing engineer's system is a dB louder on the left what you hear if your system is exactly neutral would be louder on the right.
If the OP's system otherwise sounds fine, this is what you have a balance control. for. Use it. Nothing else that has been mentioned will effect balance. Anti-skating will effect tracking not balance, same for level. Tube variability can do it. A bad connection will usually cause distortion and noise but possibly volume.