I must be thick, because it seems to me like this should be easy to do, and yet I can't wrap my head around how to do it. Here's the issue. I record (and occasionally record live shows) using a Mackie Onyx 1640. I use the aux sends for separate (headphone) monitor mixes for several musicians (5). When tracking, the vocalists tell me they'd prefer to hear themselves with a little reverb. I can add that in my DAW and use the monitoring function there, but that creates too much latency and drives them crazy. So I wanted to do it with hardware, which is what I love about the Onyx to begin with, total control over individual monitor mixes and all via hardware for zero latency monitoring.

As a result I've been thinking of picking up an external effects box, either a multi-effects unit, or just a stereo reverb unit. Then I got to thinking that it would be nice to use that same unit to add a little reverb to the drums as well, just for monitoring (we tend to rehearse through the phones too). So in the end, I want to use an external reverb unit to add some reverb to 2 vocal channels and 7 drum channels on the mixer at the same time, just for monitoring purposes. I'll add proper reverb later in the DAW to create the finished product (and the Onyx wouldn't record the reverbed signal anyway, the out to the DAW is post-gain and pre-everything else).

If it was just the vocals with two vocalists and a stereo reverb unit, this seems easy, just use the channel inserts/returns on those two vocal channels, each channel sending to one of the two reverb processors and returning to that same channel. No problem. But surely it's possible to set up a single reverb processor and then have multiple channels on the Onyx feeding into it, and perhaps even dialing in more or less of that reverb on individual channels? That's what I want and what I don't understand how to do. Would I use one of the aux sends/returns to send each track to that reverb unit and adjust the send level on each channel to control how much of the reverb effect gets applied? If so, I'm using the other 5 aux sends for headphone mixes - how does that signal that has returned from the reverb unit then end up going back out to those 5 headphone mixes, i.e. how can the reverbed vocals (or drums, or whatever) be mixed at diffferent levels for different monitor mixes using the remaining aux sends so that each monitor mix can have different levels for vocals, drums, etc? I'm not trying to control how much reverb there is for each monitor mix, just how much vocal level there is in each monitor mix once the reverb has been applied to it, if you know what I mean.

I think it would be easy if I just had one monitor mix for everyone (or if I just wanted that reverb sent to the main mix for a live audience), I would just assign the aux return with the reverbed signal to the main mix and/or to a single monitor mix by pushing a button. But how to do it when there are multiple monitor mixes being created through all the remaining aux sends? Is that possible?

All those gozintas, gozoutas...it seems like it should be possible, but I can't figure out how.