Take a look
here for some decent pictures. It does seem to resemble Hotblack Desiato's stuntship from HHGTG.
This is a very interesting headamp, with a different topology and schematic then regular headamps tested here. It's hard to measure like OPA1612/LME49600 amplifiers for sure, but I really appreciate it's bold design.
From what I see, there is a pair of IRF510/IRF9510 on each channel, so probably by carefully adjusting the bias current might decrease few dBs the amplitude from the 3rd harmonic. However, I really don't think the first couple of harmonics will be ever audible by any regular humans if they're at least 90dB away from the fundamental sound, so don't think worth the trouble, especially that the output DC might suffer.
However, I see no transformer inside the case, so perhaps the mains hum comes from the surroundings, or maybe the volume pot or the outer case is not properly grounded; sometimes the audio ground is better to be separated by the mains ground, so not sure what's the designers approach on this one, but worth "investigating" by simply touching the outer case with your hand, then touching the volume knob, then touching the RCA/XLR ground as well and see if the mains increases or decreases. Very sensitive IEM's can be used, but also a regular sound-card connected to headphones-out as well.
Of course, AC ripple can be measured with a scope around the power regulators (yes, same
IRF510/
9510), just in case this main noise might come from in there. Not sure how the
voltage rail splitter was done here, due to the
IPB120P04P4L MOS-FET from the input (just after the power plug), but I'm guessing there's sort of dual power supply built around the 2 x IRF510/9510 MOS-FET pairs. So, the mains hum might be from this rail splitter, but it's impossible to know without a scope. Perhaps the input transistor (after the power plug) acts in a similar way like the
TLE2426 or like a Zenner diode, but I'll let an EE to "investigate" this further.
An interesting project with same MOSFET transistors can be found here:
http://xen-audio.com/documents/f5ha/F5-HA Description V1.4.pdf .