tiglath wrote:

I want to connect a metronome to a mono mixer track and hear the
metronome on the headphones but not the main mix.

I know the Behinger UB1202FX can't do it because the FX control is post
fader.

I see the Alesis Multimix 8USB has two AUX controls on pre-fader which
might be a way to do it. I am not sure. By turning off the channel
output to the main bus, hopefully the pre-fader AUX goes to the
headphones/ctrl-room.

I think you'd have to use an auxiliary send and an outboard headphone
amplifier for this. Just about all small mixers only send the main
outputs to the built-in headphone jack. While this seems like a simple
request, it's just not implemented on bottom-feeder mixers. You can
spend more money on a mixer with more flexible routing or you can spend
more money on a headphone amp that you can feed from the aux send.

If you have an old home stereor receiver, you can use that as a
headphone amp. Pick one up at a yard sale or thrift store for $10 if
you don't want to spend $100 or more for a real studio product.