Input Echo, External Audio and Whine [SOLVED]

Apologies in advance for the long-winded description to commence now.
 
My signal path for two external synths is this (the same for both):
Synth --> DI Box via TS cable --> Focusrite audio interface mic preamp input via XLR cable (one synth is on the Left input and the other on the Right input, both mono)
 
With Sonar running, and the input echo activated for both synths, if monitoring at mid-to-high volume, there is the "typical" hiss but also an additional high pitch "whine".
However, when Sonar is not open and I have both synths turned up and the audio interface turned all the way up, there is very little hiss and no high pitch "whine" and my monitors are relatively quiet.
 
Now, to make things even more interesting, if I unactivate one of the input echos of the two external synth tracks, the whine becomes even more pronounced.  Then, since I work with separate MIDI and audio tracks, if I mute the associated MIDI track of one synth in order to record the other synth's audio and MIDI tracks (thus avoiding triggering the first synth again) during playback/recording, the muting seems to contribute to the "whine".  If I'm using a patch with a longer release/sustain (i.e. pad), the whine is noticeable in the recorded audio.  Ugh.
 
I have found that the "solution" for now is to prepare my MIDI tracks for each synth and then record both at the same time with input echo activated for both.  That seems to eliminate the annoying "whine". 
 
Has anyone else experienced anything similar?  I know it's probably a longshot, but just wondering if any of you with advanced knowledge of audio signals, etc. would have an answer as to why there is a difference in noise level between Sonar running and not running.  I'm not wanting to monitor at the highest possible volume, but am concerned with obtaining as clean as possible of an audio signal and am curious as to why clicking on a software button might cause these differences.
 
 

Cactus Music
What might be happening is a feedback loop from some sort of crosstalk in the interface or?
Mess with the mix control software/ DO you have version 1.8? it got rid of some bugs.
 
Not sure why your using a DI box, you don't need that in the signal path, Your just adding posible noise.
I have a 6i6 , plug the synths TS patch cord directly into the front and using mix control, set input to "instrument". That is your DI box built right in.
Or use the back inputs. I have my Korg sound modul or my Digital drums plugged in the back and the level is always matched up nicely. 
 
To test your system, unplug the usb cable and listen with the headphones cranked up.
With Sonar closed , plug in the USB cable and listen.
Now open Sonar and a blank project and listen
Now insert a midi track and arm it for record and listen.
If they have both try them out.
Insert a soft synth and so on until the noise starts.
But there is a good chance this is in the Mix control software leaking from your output to the input at a very low level.
Are your synths midi or USB conection?

Thanks for the reminder of MixControl 1.8.  I'd seen that was available awhile back, but had forgotten about it.  Installed and it seems to have helped.
 
I have a DI box primarily for the ground lift that it provides.  I did try direct inputting both synths using some short TRS cables this evening and that did seem to eliminate most of the whine I'd been hearing, so I may convert to that signal path once I get some longer cables.
 
But then I reconnected my DI box setup and tried your suggestion of opening a blank project.  I have both synths setup as track templates with associated MIDI tracks, so I inserted new instances of each.  As long as I have both input echoes engaged, there is now no whine at full volume.
 
Weirdness.  But as long as it's now working quietly with either audio path setup, I'm happy.