Just recently got this amp and have read the woes and wows about it -- from my view the wows far outweigh the woes. I have the version with the internal mini switch for selecting headphone impedance (yes it's in there, lol). Anyway, I'm using this mostly with headphones (Hifiman HE-500s - 38ohm). While experimenting I discovered one thing -- THE ELECTRONIC SOURCE OF THE MUSIC MAKES A DIFFERENCE IN WHETHER THE AMP HUMS OR NOT.
i) Playing lossless tracks from my iPod into the amp via headphones produces no hum or hiss from the amp. It is black as night and quiet - a zero hiss background.
ii) Playing tracks via computer usb/dac or spdif picks up background interference. It is definitely coming from the computer NOT the amplifier. I even tried different DACs with essentially the same result. Somehow or other the amplifier is very sensitive and picks up electronic interference - I don't know why. With the HE-500s it is barely noticeable, you can just hear it when there is no music playing. With Grados (SR80s - 32ohm) the hum is more distinct - but it's funny - the Grados sound just fantastic with the amp. The mini switch is initially set for low impedance cans and it does reduce the interference.
I do not think this hum/hiss is mostly to do with tubes but rather with the electronic source of the music. I wasn't that surprised when I came across BmWr75's post -- see highlight below. Granted there are other factors, eg 600ohm headphones, but I think it has something to do with the initial source.
Anyway, just my 0.02 -- it may be worth considering the initial source as contributory to distortion issues and not the amp itself.
All that aside this is a fantastic amp, looks great too. The fact that I can drive a Hifiman phone with it is a real bonus. It sounds very detailed and warm (oh dang! maybe I'm just feeling the heat from the tubes). I've rolled some Groove Tubes power tubes in there and they sound much better than stock. Oh yeah, another thing, that mini internal switch in there is like a mini EQ (don't know whether it's worth cracking the base open for though!).
I've never had a hum issue with this amp, but have been using it with 600 ohm Beyer cans.
Just for grins I decided to see if the amp hums with high sensitivity speakers. Bottom-line - this specific amp is not hum prone with speakers.
To summarize my tube comparisons:
Amp = Musical Paradise MKII
Source = iPod playing Apple Lossless files via an HRT iStreamer DAC
Speakers = Klispch KLF-20s (100 dB efficient) with Crites titanium tweeters and Crites crossovers
Note: the hum heard on two sets of tubes below is not dependent on volume setting. The barely audible hum is the same an min and max volume.
Sylvania 6SK7GT - most hum of any tubes tried, but would not be noticeable from >3' away from the speakers and acceptable to my ears.
Tung-Sol 6SK7GT - less hum than Sylvanias. Sounded good.
Red can 5693 - no hum, noticeably more gain that the other tubes tried, sound great driving the speakers.
JAN Phillips 6SJ7WGT - no hum. These are the best sounding tubes I've found when using 600 ohm headphones. Was not as impressed with them driving the speakers, but still sounded good.