I highly doubt it is the barts. I have found the OBP-3 to be much smoother in the mids than you would expect. You can look at the wiring diagrams on aguilars site and play with the center freq for the mid voicing. It makes a big difference with that pre. I also agree with the descriptions of the Q6 with emg's. They dont sound bad, but they are not growly and punchy at all.
Zon Legacy Elite 6, Aguilar DB680/DB728, Mesa Powerhouse 112.