We have the PS Audio GCHA in the lab, and I borrowed it to try with the Sennheiser HD800s and some other 'phones.
This is a lovely sounding amp with the HD800s. Bass is solid and high frequencies are very smooth. I would characterize the GCHA as slightly warm, but this may be more of a comment on the Sennheisers.
The other thing to like about the GCHA is the excellent manual. This is especially helpful since the GCHA has a USB input (not used for this test) and computer hookups are inevitably more challenging than analog audio connections.
More as time goes on.