Help with VTL TL 5.5 Preamp

I recently acquired a Vacuum Tube Logic TL 5.5 preamp. It utilizes 6 tubes: (2) x 12AX7 and (4) x 12AT7. In trying to get to know this preamp, I've rolled several different combinations of tubes, BUT I cannot get the highs to sound smooth and "sparkly."

The preamp images well, has air and texture, is great sounding in the mid-range, good bass slam, etc., but the highs are driving me a little nut... sharp, edgy and too much grain. The highs can get in your face and blow you back a bit.

I've used various combinations Mullard, Telefunken, Mazda and Brimar. Right now, my set up is: (2) Mullard 12AX7A and (4) Brimar 6060 (T-Logo). All the tubes I've tired have been matched.

Any thoughts, suggestions or experiences?