Well, regarding the USP-1 vs. PT-100 comparison, given the information supplied by auxinput, between these two choices I would be inclined to purchase the USP-1 due to design approach/build quality. I agree that 10 years does not represent 'old' for an amplifier. Provided the condition of the USP-1 under consideration was in good condition.
There are other used preamps that could also be considered. Makes from NAD, Rotel, Jolida, Vincent, B&K, Hafler to name a few. I would look for a 2-channel pre.
Does your receiver have pre outs? If so use it as a pre and save additional funds for a more substantial upgrade if so desired.