Room gain low frequency anomaly

I tried out my Stereophile test discs. The low frequency tones are 1/3 octave warble tones at -20 dB at 200, 160, 125, 100, 80, 63, 50, 40, 31.5, & 25 Hz (maybe 20 Hz, too, altho it don't matter to my spkrs). I tested it on my Polk AW spkrs ("B" spkrs on my stereo rcvr), my MB Quart QL S530 bookshelfs ("A" spkrs on same), & my el cheapo HT in a box (I am too ashamed to identify it, but in self-defense, I'll say that it's fairly priced & Bose would charge twice as much for the same thing). "A" spkrs are placed correctly for 2-ch bookshelfs. "B" spkrs (Polks) have drivers tilting upward, so they are on carpeted floor below the "A" bookshelfs. The front L/R HT cubes are on top of the "A" spkrs with the bass module on the floor beneath the TV shelf.

Overall, response was smooth & consistent from spkr to spkr. The Polks started dropping drastically right when they should (their - 3dB range is 79 Hz), the MB Quart did the same (specs give 44Hz as bottom range, no +/- dB given), & the HT in a box had the expected gap in the 100-200 range (the 125 Hz tone was noticeably weaker than the 160 & 100), where sub & cube don't quite meet (crossover not adjustable on this model).

Here is my issue--all 3 sets of spkrs seemed to "jump" at 50 Hz. This was hard to detect on the Polks (since they aren't even rated that low), but it was there, and was more obvious on the MB Quart & the HT in a box. The 50 Hz tone didn't seem to warble as much as the others, but was more like a smooth rumble, & it was a little louder than the 63 Hz tone. My amateurish attempts to diagnose this led me to the possibility of a room gain anomaly at that frequency.

If this is what it is, any ideas how to fix it? Or if it's not a room gain anomaly, what could it be? A bad test tone?