Weird hum issue

[Todzilla]

I have a very minor hum issue, which I suspect is not caused by my SCA preamps, but it manifests itself on the signal chain they're in, so I thought I'd ask.

Here's what I'm hearing: When I really juice the preamps and my monitoring environment, I hear a faint hum, some 60Hz but not strictly that freq, which is clearly exacerbated by my turning up my A/D converter (MOTU 896HD)'s input levels. I can manage my signal chain to minimize to the point it is not super noticeable, although with 20 quiet vocal tracks, I have to take pains to minimize it.

The weirdest thing, which makes me highly doubtful it's the fault of the SCA preamps, is that I can hear it when the SCA is turned off.

If I disconnect the SCA from the MOTU unit, the hum diminishes with each channel I unplug.

Also potentially relevant is the fact I have a 30' snake to feed the SCA unit, which may be picking up some RFI or something, although I would assume give the SCA is off, that this would not be an issue.

What should I be testing for? and should I be suspecting a grounding problem in my studio?

Thanks in Advance