Behringer Powerplay PM1

Rated 3/5 as it is expensive for just a volume control knob. (It is not a headphone amp!)
Use:
Use as a belt-mounted volume control connected to a headphone amp/output, if you're using this on stage.

After testing, this works with the balanced mono output of an X32 (but i assume one ear would be out of phase from the other ear), but not the XLR output of a UMC404HD audio interface. (maybe this is not a properly balanced connection)

Test before you commit to buying more!

Pros:
+ no added noise to the signal
+ good build quality
+ smaller than the Behringer Powerplay P2 (slightly different use case though)

Cons:
- needs a headphone amp for use on stage.
- ideally needs a stereo signal- see description.
- price, would expect it to be cheaper for just a volume control.

I made a 2 mono XLRs --> 1 stereo XLR adapter, which works if you don't need it loud, and so may be ok in the studio, but not on stage.