Awesome how we both agree. I also tried it on the Quad PA One + amp and was extremely impressed with this pairing. The dynamics, imaging, detail, etc. were fantastic, but I did think the lows were perhaps somewhat 'syrupy'. This could change however with burn-in or tube rolling. In fact all of my best amp pairings with the HEDDphone were with transformer-coupled tube amps (AKA SET or OT amps). The tubes add a welcome touch of warmth / saturation and generally have a low current that controls the low-z requirements of the HEDDphone very well.
Which brings me to your question about designing an amp specifically for the HEDDphone. Yup I had the boutique headphone manufacturer
@2359glenn who is well known on Head-Fi for his OTL tube amps build a custom amp upon request. He also makes beautiful SET / transformer coupled amps that support the EL3N tubes, and the few people who are lucky enough to have one consider them better than the OTL tube amps, including the opinion of Glenn, the amp designer as well. Here is one of his GEL3N SET amps:
While the above should do quite nicely, I had mine customized for a 32-ohm output which should fit nicely with the 42 ohm HEDDphone impedance, along with many other of my headphones, and is said to also work with hi-z cans too such as HD-600's 300 ohms. Along with the premium Lundahl transformer and both 4-pin balanced and SE headphone outputs of the above, mine also has an 8-ohm speaker output which I could also tweak to experiment with headphones, a single rectifier tube that is interchangeable with 'plug-in Hexfred' diode for the option to give more of a tube / solid-state hybrid sound; power tubes compatible with the highly renowned EL12 and KT88 and many more tube types that I can run as 2 or 4x depending on preference, 1x 6SN7 slot which also allows many other interchangeable types, along with 1 or 2 stage function switch which bypasses the driver tubes for a more clean / dynamic sound option, and even a cathode resistor bias switch that was recently concocted to allow better operating points with tubes it wasn't implicitly designed for, and upgraded internal capacitors for optimal performance.
I'm hoping to have the amp in hand in around a month, but I'll have to use it with my current selection of open / closed dynamic headphones until I can afford the HEDDphone. Needless to say, it may be a while until I can post impressions of its' pairing with the HEDDphone. If you're interested you're more than welcome to post on
this thread, but take note the above is a 1-man show who works as a labor of love part time, and the waiting list for one of his amps is around 6 months.