Firstly, the PMC TB2S+ (passive, upgraded tweeter) is different than the TB2SA (active, Bryston power pack). Please verify with PMC. Amplification of the passive version is a significant component of TB2S+ sound and performance. Having said that, I've heard the TB2SA (active, Bryston power pack) and ProAc Studio 100 (with Bryston 4B Stuart Taylor edition) several times.
My impression is that they are very close in performance. IMO, the ProAc's are *slightly* more accurate whereas the PMC are *slightly* more detailed. IMO, the low-end was *slightly* more tight and punchy on the PMC (indeed the woofer excursion is so fast it is imperceptible by the human eye) whereas the low-end was *slightly* less defined on the ProAcs.
Again, these two are very close and both excellent. Room positioning also may be critical consideration as the sound and response of both are significantly influenced by their positioning. AFAIK (please verify) the PMC are rear-ported and should not be positioned close to a wall. Whereas, the ProAc is front-ported and may be positioned closer to a wall.
Please inquire as to the robustness of the ProAc woofer driver as it *may* have been improved upon.