Reply to Channel balance issue

Hi, I have a strange problem with my turntable setup - my left channel sounds like it has a lowpass filter on it, this happens on both decks and on every record.
The volume is pretty much balanced, but one channel just lacks treble.
This does not happen when I listen through headphones plugged into the mixer.
My setup is 2x1200 Mk2, Ortofon Concordes, Stanton SMX-401
Is this a problem with the mixer outputs?
Thanks!