Don't you have line outs of the mixing console or aux out of the mixing console so you do not use the headset amp except for yourselves (leaving that hooked up like it is) and use the line outs or aux out (stereo probably) to hook up to the USB device that should have a line input also if it has a mic input and some have also electronics for a lower signal like a guitar high impedence input all done automatically on the unit or through a volume or gain control on the unit to set on the USB device?
"The laptop I use has no mic or line in, so I use a usb sound card. The usb audio has a mic input."
I never known any sound card whether professional or not, not have a mic and also a line input - usually a regular gaming type sound card (or built in ) will be 3.5mm mini-stereo plug and not the usual 1/4" guitar cord (but in stereo) type input or for professional connection a XLR input for the mic. I also find that adjusting the volume on the input of the sound card is how you control the volume if too loud through the OS system and mixer of say like Windows OS which is done on the computer and mixer program of the unit, although a lot of mic inputs on sound cards have an additional level to help out the mic - except that on a regular computer sound card I think it is 9 volts because mics on computer are cheap condensers and on a USB professional device it is 48Volts for a condenser mic that needs such an input and can be off or on if using a dynamic mic (or ribbon mic which may take more gain then most units provide).
So to me, a professional USB device would have mic and also line inputs either set by a volume knob or gain control knob on the unit unless it is one that is done automatically (which also works but does it by the unit its self) and a combination Neutrick connection is used, can be either a guitar input, a line input or a XLR mic input on the unit.
An example of such a unit, and there are many USB devices:
http://www.sweetwater.com/shop/computer-audio/audio_interfaces/ Typical example of a USB device:
http://www.sweetwater.com/store/detail/Scarlett2i2/see the combination input connector and gain control on the unit, that is what a line out of the mixing console would be plugged into. Are you sure the headphone amp also does not have a line output on the amp either, where you have outputs but not for headphones (maybe a RCA phono plug out). Whether mini-plug (3.5 mm) or regular 1/4" headphone connections there would be an adaptor also that can be bought to change to the input or output you need.
http://www.sweetwater.com/store/search.php?s=3.5mm+to+1%2F4%22+stereo+adaptor&Go=Search But if nothing else, perhaps you just an an attenuator to down the volume coming out of the headphone amp.
That you will have to search for on your own, or maybe someone else can give an opinion also here.
I am sure though that something is made that will work in your case.
I just fail to see why it does not work now, if you are using the drivers and programs (mixer) on the computer that are on there either through the OS like Windows or through the drivers for the sound card whatever it is.