I drive the HD600s from my Denon AVR-5700 receiver (though I do use a dedicated headphone amp for portable use).
QPSK is Quadrature Phase Shift Keying, a data format where two bits are sent every symbol time, where one of four signal phases encodes the data. EFQPSK is Enhanced Feher-patented QPSK, a similar format that's designed for higher bandwidth efficiency. The beauty of these formats (particularly EFQPSK) is that they have a unique signal pattern when viewed on an X-Y oscilloscope - unfiltered EFQPSK is a perfect
circle on an X-Y scope. Any distortion, phase error, DC offset, or level mismatch is instantly recognizable, even errors which would never be audible on sound material to a mere mortal!
By the way, you're right about my reference being Gaucho - I'd accidentally said Aja above, and corrected it just before you posted. To compare the "live" DTS encoding to the pre-encoded version, I'm afraid that I don't have any test instruments other than my ears!
To perform that test, I'll use the Advanced Audio version of TheaterTek Version 2.2.1 to internally decode the DTS audio, and then I can just switch between the card's 24-bit, 96 KHz analog connections, and the real-time DTS encoding connection over S/PDIF. I'll also try pure DTS-passthrough over S/PDIF for completeness.
I've not used Lynx or RME cards, but I have used M-Audio's sound cards for several years before moving to the XMystique, so that's my standard reference.