Sub-Bass exacerbating Cross-talk?

A fellow reel-to-reeler is experiencing cross-talk across the board on his five, open-reel decks (4 TEACs and an Akai). He’s recorded with three different machines and hears the same reverse track(s) cross-talk in the 4-channel playbacks on all five machines. Because it seems unlikely that all five decks would have identical misalignment issues, either in tape path or head azimuth, I asked a bit more about his system and the nature of the cross-talk.
The cross-talk manifests predominantly in low frequency, and quite possibly ultrasonic on the low end. His output is through a theatre sound system and it is through that system’s sub-bass woofer that most of the offending signal is heard. (Diminished cross-talk still can be heard through his main speakers.)

Has anybody else experienced, what I am assuming are acceptable levels of cross-talk exacerbated to unacceptable levels with use of a sub-bass in open-reel play?