Brad, thanks for your response on the limits in your area. Presuming even flow through each of the three suction lines (not necessarily so), I would never exceed 6 fps there. If I can go to 10 fps on the return, that gives me more pump choices.

Mark, I don't have a spa, so, I've been comparing curves for high-efficiency 1/2 hp and 3/4 hp full rated pumps (with service factors of around 1.9 and 1.67 respectively). The Sta-Rite Max-E-Glas, Sta-Rite Max-E-Pro and the Pentair Challenger seem like good choices with "steep" pump curves that could still move water as the filter gets clogged. The clean filter pressure is about 18.5 lbs. which translates to 43 feet of pressure head, I believe. I have not measured the suction head but guess it's around 5 - 10 feet. Does that make sense? Unfortunately, I'm away from my pool for several weeks, so can not give you the details on the current pump.