How can the E-trap add audible distortion at higher frequencies when (1) it is not in the signal path and (2) it is sharply frequency limited by design?
(1) It is in the signal path in the sense that it is making acoustically audible signals in your room (although these are 'corrections' or 'damping' there is every possiblity that other detrimental audio is generated)
(2) The amplifier circuitry is no doubt sharply frequency limited by design....however a transducer is NOT. Most of what you hear from an impressive subwoofer is harmonic distortion...that is the intended signal at 30 Hz is actually nigh close to inaudible (due to fletcher Munson curves our hearing is pretty insensitve there) but the 20% harmonic distotion at 60, 90, 120 and 150 Hz is absolutely impressively deafening!!!
Think twice next time you hear an impressive subwoofer - with the majority of them you are simply hearing huge amounts of distortion.
So lets assume the bag end has an impressive 10% harmonic distortion and you use it to fix a 30 Hz ringing problem....what happens at 60 Hz and at 90 Hz (where you might have a NULL and need a boost not a cancellation)....that is my point!!!
The nice thing about broadband passive absorbers is that they add NO harmonic distortion...