I'm using a Roland Studio-Capture (and also have an Octa-Capture that works the same way) and sometimes I get singers that complain about not being able to hear themselves in the mix.  I have the singer's channel maxed out in the Direct Monitor mix in the Studio Capture and the instruments generally at "0" level.  I can lower the instruments so the singer can hear better but then the other musicians complain that they are not loud enough. 
 
Is there another way to get "more me" to the singer and still maintain useable instrument levels.  Not all singers complain about this but I've had "more than one" so it is a problem.
 
 

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.

I bought a cheap Behringer headphone ampl because I had the same problem with my Presonus 1818vsl. Surprised how good this is for $25 http://www.musiciansfrien...mp-ha400-headphone-amp

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??? 

I don't have a mixer with 4 outs, I have one with 2 outs.  I'll experiment with that and see how it works. 
 
It has to be the singer, though.  I did another recording session for the guy and we did 6 songs and he didn't complain about not hearing himself.  I set the Studio Capture Control Panel (same for the Octa-Capture) Monitor Mix with the channel for the singer at Max (+12db) everything else at 0db and he was happy with the mix.  We had SR-16 drum machine, Pedal Steel Guitar, Lead Guitar, Rhythm Guitar and the Singer's scratch track.  I have to add in bass and possibly some keyboard.
 
My headphone amp is a Behringer Powerplay Pro 8 (8 channel).