Another good idea but how do you instantiate a reverb? I can only hear mine on playback.
 
I have an external unit (TC Electronic M2000) but I've only had limited success routing out to hardware effects.

You Traveler has a reverb. It's not the best but it works. I happen to use one from time to time because my friend has one. I think really the "least fuss" solution for you would be to create a send on your master bus to outputs 3/4 (or to a bus with those outputs), set the headphones to 3/4 in the CueMix app and in the CueMix mix for outputs 3/4 add the mic input channel. That way the singer will get exactly the mix you already have, the volume is determined by the send on the master bus and any changes you make will be on the singers ears too.

I don't think my Traveler has a reverb. I have the older one, which I think is obsolete now, great unit though.

Beepster
You could also just do a "Save As" and totally mangle the project in any way you want as you track the vocals (or whatever) then do a dry track export (no effects) of the tracks you recorded and import them into the original project for mixing.
 
Tons of ways to do this stuff which is what is so awesome about digital recording (one of of the million and one things that is so awesome about it... lol).
 
Cheers.

Yeah I've done this as well - I had one project on which V-Vocal would not load without crashing the project, no matter what I tried. So I exported the mix sans vocals, imported it into a blank project and tracked the vocals from there. Did all my V-Vocal work, then exported the treated vocal tracks back to the main project.
 
The only thing to be mindful of was the tempo maps had to be identical

Bristol_Jonesey
The only thing to be mindful of was the tempo maps had to be identical

Oh... yeah, totally but the Save As of the original project should retain all that type of data. Then freezing or bouncing or archiving or deleting or whatever needs to be done to make the project lighter on resources can happen for tracking.
 
It is certainly a dicey affair when using things like V-Vocal/Melodyne/Audiosnap/clip "stretching"/etc in large projects so even if I'm not tracking I tend to make sure I've done a Save As to see how the project is going to behave and act accordingly after that (which includes exporting/importing the results of such procedures back into the original project).
 
Actually I may do full exports of ALL tracks multiple times and import them into new projects over the course of a project. Like I'll do all my writing, edit/bounce/flatten all my "scratch tracks", import into a new project for final takes... then edit/bounce/flatten that and bring it into another new project for mixing.
 
Otherwise I'll do a ton of Cloning and Archiving as I work if it's not TOO nutty of a project (or if I'm just doing a few tracks for someone else).
 
But that semi sorta OT and I'm semi sorta insane.
 
lol
 
Cheers.

revnice1
...giving the vocalist more level so she can hear herself. Turning up the headphone volume raises everything at once...
...I can turn down all the other tracks...

 
If you use a pre-master bus for the backing track, i.e. all instruments and everything excluding vocals, you can easily turn this down for recording vocals and turn it back up afterwards. This might actually be the quickest solution.

Here's a short story about what worked well for me:
 
I had a room full of vocalists, and despite having a pair of OZ Audio HM-6's it still was a chore to try to provide useful headphone mixes for everyone without breaking the vibe of the session.   
 
I finally invested in inexpensive Behringer MA400 headphone/mic/cue mixers (monitor headphone amplifier) and made brackets for each MA400 to attach to a microphone stand.  Now, each vocalist can get a generalized mix sent to their MA400 that allows them to set the cue mix level in their headphones while also allowing them to independently set the amount of direct feed from their individual microphone into their headphone.  The MA400 uses a 12V, 150mA wall wart.
 
The microphone connects directly into the MA400, and is internally jumpered to an output XLR that goes to the preamp that's connected through the A/D interface to SONAR. The phantom power (as needed) comes from the A/D preamp through the internally jumpered connection to the microphone. The MA400 has an internal mic preamp that taps the jumpered XLR connection and feeds the singers headphone. This also means that there's no/miniscule interaction between the MA400 and the expensive/desirable A/D preamp that feeds SONAR. A second unbalanced stereo TRS input to the MA400 is for the cue mix that the singer uses. 
 
There are other products that do the same thing, but since I needed seven of them, I needed to keep capital costs down. A bit of caution is necessary. I did have a situation in which a singer over-boosted their direct microphone knob and therefore sang too softly.  I now advise singers. to first set the cue mix level and then work up from zero on their direct microphone knob. They like having the immediate real-time control.
 
HTH

revnice1
I don't think my Traveler has a reverb. I have the older one, which I think is obsolete now, great unit though.

Oh sorry I work with a Mk3 from time to time. In that case your only options are patching in an external hardware reverb or monitoring everything through Sonar. In the last case you will need to get your latency down to an acceptable level, so it really depends on your system. For a hardware patch you could use CueMix to direct the incoming mic signal to another output, throw that through a reverb and patch it back in to the original headphone mix.

cryophonik
I'm sure that people have different preferences here, but I personally NEVER record the singer into the original song project.  I export the song to WAV and import that WAV into a new project template that is already set up for recording vocals.  My template already has tracks inserted for the guide track, a folder full of ready-to-go tracks for the lead vocals, another for harmonies, etc. all pre-routed to busses, with a reverb already instantiated for the headphone mix, etc.  That gives me a lot more flexibility for controlling the headphone mix and latency is a non-issue.  I also use that project file for comping vocals, pitch-correction, etc. to clean up and finalize the vocals.  Once the raw vocals are done, I bring them back into the original project for final mixing.

+1  what I do too except I do all comping and correction here too before bring back to the  main project. just easier.

Once you have a hardware set up for cue mix,, it is always there and there's no need to waste time mucking about with your project. It's dependable and fast. And does not have to cost very much at all.
And by hardware, I will also include if you purchase the correct interface as many are capable of multiple cue mixes with reverb.