audio interface for mixing only - need recommendations please

Not the Roland Quad. Works well with Sonar, but the Headphone/master out is annoying

The line outs are on Phono plugs.

They are usually unbalanced, which can cause noise problems, coupled with a drop in volume.

 
It is best to have outputs that match specs and connectors with the inputs.  RCA and some 1/4 inch are unbalanced.  As Mr. Jonesey sez, unbalanced cables are more prone to interference and noise.
 
You haven't said how much you want to spend.  In general, interfaces sound the same in a price range.  What you want are drivers that work with your specific computer.  All should work, but there are times when your system has conflicts with a specific unit.  So you want to be able to return a unit.  You problems are slight w/ drivers, since you don't need low latency, just compatibility.  I would think any music interface would work.  In your situation on the low end I'd try the new Tascam units (Gibson owns them and Cakewalk so you figure the drivers should be well tested w/ SONAR).  Steinberg (made by Yamaha) would be high on the list, as well as Focusrite (many happy users here).
 
On the higher end I would go for the Tascam UH-7000.  High end conversion - better than my TC electronic interface (which is "pro" level).  Very clean and pristine, which is what you want in monitoring, like studio monitors.  High latency, which ain't a problem for you.  Much better preamps than prosumer interfaces.  I know that isn't what you need, but if you ever do ....  Great buy for $400, if you want to spend that much.  It is good enough you won't ever need to buy another interface, even if you win the lottery.

guy here complained of real problems using X3 and the tascam unit in his review

http://www.amazon.com/TASCAM-US-2x2-USB-Audio-Interface/dp/B00MIXF2RS/ref=sr_1_1?s=musical-instruments&ie=UTF8&qid=1427295407&sr=1-1&keywords=tascam+audio+interface


I dont know, maybe these reviews are just one in a million problems... but that one was specific enough to spook me
 

If you're looking for top fidelity (and ultra low round-trip latency isn't a priority), the Tascam UH-7000 would be a good fit.  Great converters.  Very nice preamps (if you ever did need to over-dub).
It's ~$400, but it offers fidelity superior to anything else at that price point.

Anything RME is exceptional.
Of course, the cost reflects that.   

A smaller Focusrite (mains powered, not one powered by the USB port) or, depending on budget, RME Babyface would be my choice. Maybe a Roland.

Monitor outputs really do need to be balanced though (assuming the monitors accept a balanced input) which means TRS jacks or XLR connectors.