Jeff's answer is spot on. There are other places to change frequency response, but do you really want to do all the math to calculate effects of changing cathode resistors and bypass caps, or bypass caps on the plate resistors, or changing the coupling caps? Changing that one cap will do a lot to the mid response. You might want to buy a few values and experiment, see which one you like the best. 100pf is a great place to start though!
I personally *like* the mids in the XTC, but you may not need the same amount of 'out front' that I like when I gig. Tones that sound better to me on their own sometimes get lost at a show, and I primarily use my MTS stuff to play out with.
Pete