This is sort of...odd. How could you be hearing the LFOs unless they're running at audio frequency rates? If the "LFO sound" is actually more like a constant hum, then the problem probably isn't the LFOs at all and you're dealing with a ground loop. The big clue here is the phrase "older, ungrounded electrical systems"; grounding is ESSENTIAL for electronic instruments, processors, and the like, and if it's absent, the equipment will seek a "ground" by whatever means it finds. In this case, it sounds like it's found your A-143-3...and that's not good for that module if that's really what's going on here, because it's passing leaking AC line voltage back thru the module, the cab's distro and PS, and the like.
The practical way to disengage a ground loop is to add isolation to your modular's outputs...which is something EVERYONE should be doing for exactly these reasons, particularly if you play live and you're plugging into power systems you know nothing about. The Lifeforms Outs you have doesn't appear to have this; I would strongly suggest looking into an output module that has proper balanced TRS mono outs, not TS. Furthermore, if you make sure and find an output that's specifically transformer-isolated, this will not only give you the isolation you need, it also puts some "iron" into your output signal path that you can saturate a bit with a touch of overloading that'll give you a warmer signal provided you don't go overboard with the levels.