Bill - This shouldn't be complicated. The Focusrite 2i4 doesn't come with Mix Control, so I assume you are feeding the headphones with the
same monitor mix that you use for playback and mixing, right? If so...
- Set up your mix the way you like it
- Create a new bus (by default it outputs to your master bus)
- Put a reverb plugin into the FX bin of the new bus; set the reverb to 100%
- On the vocal track add a send and point it to your reverb bus (careful - a new send defaults to "too hot," so turn it down); this send should not be set to "prefader"
- Arm the vocal track and enable input monitoring (speakers off!); if your mic is plugged in and routed correctly you should now be hearing your backing tracks and the vocal mic in your headphones
- You can adjust the mix -- the blend between vocal and backing track -- without affecting what gets recorded; using the faders in console view, balance it the way you want it and add some reverb on the vocal track using your new reverb send (the reverb won't be recorded, but you'll hear it in the headphones)
The latency using this technique should not be a problem except for the fussiest of vocalists. Get your buffer as low as possible and avoid using a bunch of plugins while you are tracking, as some can increase monitor latency too much.
This works for me, but of course, I work alone. I don't need separate headphone mixes as I am the producer and the singer. If you do need separate mixes, it gets a bit more complicated. Good luck!