For some time, I've been running XP with a Diamond soundcard connected to a USB DAC with fiber optic, not USB, because the USB connection was slow/spotty. I also had a pair of A2 desktop speakers connected to the 3.5mm jack in the soundcard, therefore I could listen to either the desktop speakers or my Denon 2000s, simply by turning them on/off.
 
I recently upgraded to windows 7 and immediately tried the USB connected to the DAC, and it worked great! However, as you probably are aware if you are reading this, the USB DAC essentially shuts the soundcard off, so I no longer can listen to my desktop speakers with the USB DAC plugged in with USB, however, I am sure it would still work with the fiber optic connection.
 
 I do not wish to unplug things when I switch between my headphones and speakers. I am hopeful that someone might explain what is going on, and if there is a solution to this issue other than simply going back to the fiber optic connection.
 
My soundcard is a Diamond Xtreme SX51:
http://www.tigerdirect.com/applications/SearchTools/item-details.asp?EdpNo=2177471&CatId=2769
 
My DAC is a Super Pro DAC 707:
http://www.onheadphones.com/product-reviews/dac/a-small-purple-dac.html

You can use RCA splitter adapters and connect both your speakers and your headphone amp to the DAC simultaneously. You speakers may even sound better after that too.

Thanks Yoga Flame! Why didn't I think of that?!
 
I'm wondering if that would impact the SQ negatively for the DAC>Amp>Headphones? Or how many times can you split the signal before it degrades is some way?  For example, what is the limit to splitting the signal, because by this logic I could split the signal a hundred times or more.
 
I think your suggestion is an excellent one...this question is more of a theoretical nature.

I searched for a bit and came to the conclusion that is presented in this thread:
http://www.head-fi.org/forum/thread/66163/rca-y-splitters-cause-any-signal-degradation
 
It should not cause degredation that is noticable, at least not if the cable length remains relatively short. 

I was just able to achieve this by using a virtual audio cable, ASIO4ALL, and a vst host.  I run Cantable Solo as a VST host, but it should work with seibs free VSTHost / Cantable lite or the like.  A VST host may not be required/preferred at all, any audio software with routing capabilities that supports ASIO (should) do the trick.  But I already have Cantable installed and was sitting here doing nothing.
 
Using VAC, create a virtual audio cable on the system and configure it to be the default playback device.
Using the ASIO control panel inside the VST host, set both your USB device and the internal sound card as active outputs.
Configure the pin outputs on the VST host so that there are two sets of L/R pins, and configure one set for the onboard sound device and the other set for the USB sound device.
 
 
Verdict is still out as far as if this causes clocking problems or other SQ issues.  Based on my quick 5 minute test, I think it is probably OK but more testing / tweaking would be required before I would say it is a competent solution.
 
You may be tempted to simply create a virtual audio cable as a default playback device and then use KS audio repeaters to duplicate the stream to both output devices.  Using KS I have serious problems doing this, MME also seems hit or miss.  I thought there were clocking problems, but I'm thinking it is something else now.  It is worth at least trying I suppose but for my system at least I'm not getting good results.
 
Also FWIW anytime I configure ASIO4ALL to use multiple input/output devices simultaneously a reboot is required on my Windows 7 installation.  Shouldn't be that way, but it is for me.