I know this is a lame question, but I can't figure out the easiest way to do this.

I'm trying to set up a headphone amp for artists in the tracking room, but I made a mistake when ordering the headphone amp and forgot it was a stereo amp. The sound is currently only playing through one ear of the headphones. It doesn't matter to me if the signal is mono or stereo, but I'd prefer to only use one of my four send channels in my snake. My signal chain is currently:

Pro tools mono bus -> mono output -> TRS-XLR -> XLR (snake) -> XLR-TRS -> Behringer HA400 headphone amp (stereo input)

What would be an elegant or simple solution? Again, I would prefer only using one channel of my snake for this. I have also tried a single TS cable from my direct out to the amp and it still only plays through the left channel.

p.s. Out of pure curiosity, how do you guys handle headphone monitoring?