I noticed you have everything already running through the Xair18. The latency through the Xair for mission-critical processing (the initial EQ/dynamic shaping) is basically zero. I mean, nothing is totally zero in the digital world, but to keep it simple, it’s zero.
The moment you start offloading that processing for vocals, you risk breaking the connection between performer and voice (if you are going to be monitoring from GP and not direct from the xAir).
If you really feel you need additional processing beyond the boards capabilities, and updating the board is out of the question:
For live performance, your most handy tool in addition to the onboard EQ/compressor will be a zero latency dynamic EQ like Toneboosters or ProQ3, or a zero latency multiband compressor. More than enough power to tackle voice resonances, plosives, and Sibalance … but also enough power to destroy your sound. Put post the Xairs processing, it’s also a good “shaping” tool post compressor.
The only “color” compressor that I add live would be an 1176, Dbx, or LA2A emulation. That all depends on the character of the vocalist, and what your group is trying to achieve. My vocalist sounds awesome through the Steinberg 1176 in my bands CL3, but it’s a process hog. 1176 can add subtle saturation and keep you from adding your own… unless you need it for effect. The less things there are to go weird live, the better.
Now — if you’re offloading vocal post fx, reverb, delay, chorus the time delay doesn’t matter nearly as much. At that point, you pick your favorite plugins that don’t kill your CPU!
-Delay into reverb softens the delay.
-pre delay on reverb is your friend
-long reverbs push vocals back (without being managed)
-EQ the reverb… either into or out of, doesn’t matter
-Post fader send for effects
-multiple reverbs = more stuff to manage, but typically a “polished” sound when done tastefully
Mastering.com released full 7-10 hour courses on EQ compression and reverb on YouTube.
I thought I had a decent conceptual understanding, but did the compression one anyway, and it totally changed the way I look at these tools.