With a little thought you could easily set up your system so that your speakers are fed with a balanced signal whilst still making use of your headphone amp.
 
This should reduce any noise in the system.
 
My setup is similar to yours and the routing is as follows.
 
In SONAR, my master buss feeds hardware outputs 1 & 2. These feed into my mixer and the main outs of the mixer go via balanced cables to my monitors
 
In SONAR, I set up a buss called Headphones. This buss feeds into hardware outputs 3 & 4. These feed into my headphone amp. You can connect what you want to the headphone amp outputs.
 
All of my other busses (Drums/Vocals.Guitars/Keys etc) all have a SEND to the headphone buss, so you can tailor what goes to the headphone amp by adjusting the send levels of each buss.

Bristol_Jonesey
With a little thought you could easily set up your system so that your speakers are fed with a balanced signal whilst still making use of your headphone amp.

I had been feeding my hardware mixer into the Samson headphone amp, and then out one of those headphone channels into the monitors, and the headphone amp doesn't do balanced outputs.
 
As it turns out, my mixer has two pairs of outputs.  The 1/4" phone pair was going to my headphone amp.  The XLR pair was unused.  And the Yamaha monitors also have both XLR and 1/4" inputs.  So I simply left my headphone amp in place (with the Yamaha monitors disconnected) and ran XLR cables directly from my mixer to the Yamaha monitors.
 
Problem solved.  Now the monitor levels seem identical and the noise is lower.  Most of the remaining noise is coming from a keyboard synth that I want to hear but I never record from (other than MIDI), so it is of no consequence.
 
I had no idea unbalanced lines would make that much difference.

Good news

http://forum.cakewalk.com/Room-Treatment-Question-m3483196.aspx#3483331   Heres a thread!