More singer in mix?

I can't think of a way it would work using the direct monitoring on the Octa Capture.  What I would suggest is putting a mixer in front of the Octacapture and using it for control of how much each person gets back.  This is the way that I control the house mix vs. recording mix.  I use a Mackie mixer with direct outs for the inputs to my MOTU, and each band member input gets a send to a monitor output.  my mixer has 4 monitor outputs, so there are 4 possible band mixes.  one of them is for vocalists.  you can even put a reverb send on that monitor mix if the vocalist wants verb on his/her voice in the mix.  and all that is a separate path to the MOTU.

Ditto above advice. I bought a MAckie MIx 8 for around $60 and this is my monitor control. 
 
My set up involves a Joe Meek 3 Q pre amp and it has 2 outputs, so one goes to my interface and the other goes to the Mix 8. 
I use the 1/2 output of the Scarlett to the Studio monitors and the 3/4 goes to the Mix 8. 
So the playback and the vocal mike both both have a level control on the MIx 8. And it's beond loud! 
I also bought a Lexicon MX200 that supplies a little reverb in the Mix 8. 
I was tracking a singer who insisted on also recording an acoustic guitar which had no PU so I took that mike and I used a Y cable to split it between the Mix 8 and the Scarlett. I used the Mix 8's  Phantom power and it all worked and the guitar, nice Martin D28 recorded perfectly. 
 
I can also use the Scarletts dual headphone system but for what it's worth I still like having hardware, not software to set this stuff up. Mix Control is powerful but after 1 year I still need the manual every time I try and use it. Badly done I say. Is there anybody who has used it to set up multiple headphone cues with out reading the manual and fussing for 2 hours???