Another option for measuring frequency response is REW, Room Equalizer Wizard and a microphone.
One thing about the WOPLs is, what you feed them is what you get out. Any deficiencies in other parts of the system will be readily apparent. What you never noticed before will now be readily apparent. Hence my quest for a vastly improved PL2000 preamp.
Play some organ or cello music for test purposes. My WOPLs cleanly and accurately produce what I feed them. A 400 WOPL is overkill on a stereo subwoofer system with a MiniDSP 2x4 active crossover, and 15" Dayton Audio Ultimax subs in standard cabinets. Just punishing sound pressures playing subwoofer test tones, and the amp hardly gets warm.
If you have a bass issue it's most probably what Joe says, the output caps in the pre. I'm a believer.