I have a BFD, Reckhorn B1 and EP2500. These all have the capability to manage the subsonic range...
Not really. The BFD has no subsonic filter. The EP2500’s lowest setting is 30 Hz. I doubt you want that, even if you could measure it.
That leaves the Reckhorn. You might measure it with REW using the method brucek mentioned, and mark the dial for increments between 10-25 Hz to give you a better idea of where it’s working.
In the end this may not do much for you, because there are so many variables. A subsonic filter won’t necessarily “save the day” because its rated slope assumes a flat incoming signal. Problem is, you’re seldom feeding the system a flat signal.
For instance, say you have a 20 Hz high pass filter. A 20 Hz one-octave filter from an equalizer boosted 12 dB will essentially blow out the high pass filter and easily extend its rollout point a full half-octave (to 15 Hz).
Don’t think you’re out of the woods if you don’t use any boosting filters. Cutting filters can also be used to extend a sub’s acoustic response. Basically, anything you do with an equalizer to make your sub “dig deeper” will override a high pass filter to a certain extent.
Even if you don’t use an equalizer to extend response, you still have the issue of equalization from the program material. The movie studios mix their programming on systems that are tailored to the X curve, which rolls out the bass response below
63 Hz at 3 db/octave. That’s going to put response at their dubbing stage down 5 dB at 20 Hz. How do you think they overcome that if the producer wants some room-shaking bass? They equalize, of course, to overcome the X curve. Take that DVD to a home system that’s flat below 63 Hz you end up with another 5 dB of boost at 20 Hz for "free," on top of whatever they dialed in.
Basically, it looks like you possibly don’t have enough headroom. Short of upgrading your sub, my suggestion is to replay that Daft Punk CD and dial up the Reckhorn’s high pass filter until the sub no longer bottoms out.
Regards,
Wayne