If that preamp has internal controls for EQ, I wonder what would happen if you tried some adjustments there ? ...
The two K&K preamps that I own have a tiny screwdriver inside for adjusting bass, treble, and on one model (and one channel) mid-range. I think 12:00 is the "flat" position.
P.S. Sometimes I also get the same impression when I run my Revolution SOLO pickup into an AI Contra. Sometimes I think it sounds "better" without a preamp, and other times I think running it through my D-TAR Solstice sounds preferable. I'm still trying to A/B these two options and pick one!