I've just watched a couple of Low End Lobster's videos about the Lusithand pre amp and I'm still not sure how it would make my PJ sound. You mentioned you usually keep the tone knob to about 20-30%, I'm the opposite; with fingers, the tone knob is always full on, with a pick I will cut it a bit, perhaps 30%. I imagine that would be too harsh for you.
I'm not a fan of flats either, though the Charvel I bought today came with them, Rotosound, I think, and I may keep them on it.
With a standard pre amp I understand; if you boost certain frequencies the end result will be predictable. With this Lusithand, I believe you are cutting the top end, gradually but can also boost the gain along with that, so I couldn't predict the end result without trying it out.