The PB1i and OB1/OB1i's are three-ways while the FB1 / FB1+ / FB1i's are two-ways ( I've owned the FB1i and they are very good. The FB1 was PMC's first entry into the home hi-fi market ). PMC has many other great two-ways in the stable including the "i" series stand-mounts and the GB1i floor-stander, the fact 3 & 8 ( I currently own the fact 8 and it is stunning ) and most recently the Twenty series with both stand-mounts and floor-standers, all two-ways. Yup, I loves me some PMC's ! 
D.D.
You are absolutely correct Diamond Dog having owned the OB1i’s and now have the PB1i’s. I took a chance (pretty safe) and ordered the PB1i’s on spec as the only real change from the OB1i’s was the extra woofer which I hoped would enable me to move the PB1i’s farther away from the front wall. I wanted more low bass for organ music in particular but also to prevent a strange hum interference from my Torus RM20 BAL Power Conditioner being picked up by the crossover of the speaker that was close to the Torus P.C. even with the speaker cables disconnected. My amplifier (Bryston 4B SST) had to be drawing current from the Torus.
PB1i’s solved the hum problem and the much improved low bass with the speakers well away from the Torus. Interestingly the already great mid range and higher frequencies seemed smoother and more articulate even though I believe the only change was the extra woofer.
I was at TAVES in Toronto and spent some time talking to the PMC Canadian distributor guys sharing space with Bryston. I wondered if I could improve the PB1i’s to the Signature Edition spec which has an improved crossover which I wondered if I could install as the veneer is ugly IMO…..only light cherry for me. I’m waiting for an answer on this.
