What IS double amping?
Many people don’t realize this, but the Analog section of a Digital to Analog Converter (DAC) actually is an amplifier of sorts, with a fixed output setting. This is why, for example, upgrading the OpAmps in a Creative Sound Blaster X7 benefits the headphone output AND the RCA “Line Out” on the back.
So, technically, any time you connect a stand-alone amp to a stand-alone DAC, you’re “double amping,” and you’re only “hurting” the sound fidelity if your amp is noticeably worse than the Analog section of the DAC in some way... for example, if the amp has a potentiometer to control and lower the volume from max.
Why am I mentioning this?
If you’re using a DAC/amp with a digital volume control, such as the Sennheiser GSX1000, you can potentially connect a discrete amp to the headphone jack without degrading the fidelity to a noticeable extent, and enjoy the benefit of a surround DSP + a higher power amp + harder to drive headphone. I frequently used my GSX 1000 that way, and I would endorse it to others to try.