PMC's are my favourite sub - £2,000 speakers (the ones I've heard). I can't comment on the FB1's (a few of the other members really rate them) but the GB1's have a fabulous all round game. The GB's bristle with detail and have a great midrange and bass for the money.
What is your current system? My appraisal is based on my sub-£1,000 per component set-up [way below, in fact]. Also bare in mind the FB1's are considerably bigger, so they should scale easier.