I don't really think the ust has an adverse effect on the tone, unless it's badly fitted. I've said it before the Fishman's are seriously under-rated. I think they just need to be properly EQ's to get a good sound out of them. My favorite EQ for them is the PARA EQ by Empress.
As for the UST changing tone, I've done quite a few pickup experiments with my oldest Martin, and frankly I've never heard it. And, I couldn't rip the K&K pickups out of my guitars fast enough (they came previously installed in 2 of my guitars, one in my Martin OM15, and the other in my Martin D17M, and I before that I tried one in my Parkwood LE061 and my old Martin), I always found them boxy with two exaggerated mid frequencies that never really reflected what was going on with my guitar top. And before you ask, I have tried every recommended preamp with the right impedance for the K&K, even internally mounting their own Tri-Star (which I felt was the best of all options but still lacking). I have found some magnetic sound-hole pickups, if heavy enough, damp the top a bit, but nothing drastic. I currently have a Matrix infinity combined with a rare earth in the sound-hole of my old Martin, which is extremely lightly built, and even with all that wiring, my Martin still sounds the same when recorded acoustically. I've had this guitar for over 20 years and I'm pretty aware of it's acoustic sound. Here it is...
