I understand that the problem did not manifest itself immediately after changing to the new pre-amp? And there is no distortion through the Audiolab's headphone output OR when connecting a headphone amp to its pre-out?
If so, maybe one of the tubes in your power amp is on its last legs?
A voltage mismatch between pre and power amp seems highly unlikely, and with your Audiolab you can easily check it by setting the gain to "0dB" effectively turning it into a passive pre-amp, if it IS a voltage mismatch the distortion should dissappear.
Edit: When I did some googling to find if the Audiolab still had the "0dB" setting (I owned the same pre-amp under the TagMclaren brand) This: viewtopic.php?f=85&t=45832
was one of my first hits. I'm confused now: Did the distortion problems show up recently, or did they also manifest with your old tube pre as described in your old thread?
Sounds to me like your power amp has a problem, not your pre-amp. (Or your loudspeakers)