I bought the SMSL sApII as a stop-gap after my Little Dot MKIII failed after 5 years of use, five hours a day. No complaints there! I just don't have the money to upgrade just now!
I have invested in some great tubes for the LD MKIII and, plan to buy another one as soon as possible.
So, in the meantime I'm using the sApII with it's supplied PSU.
My front end is a PC running JRiver Media Center 21 playing FLAC files via WASAPI to a Musical Fidelity V-LINK to a V-DAC via SPDIF optical. The V-DAC is battery powered.
Headphones are unmodified Sennheiser HD 600's.
The SMSL sApII is an absolute bargain for the price. If it was made in the USA or Eurpoe then I'm sure it would cost 3 times the price!
The bass is slightly lacking in precision and depth and there is some harshness in the mids and treble when pushed slightly. Soundstage is okay but nothing special.
I really don't know how SMSL can make these for the money?
I'm sure that this would be a very good place to start for a cheap entry level headphone amp. I rarely go above 25 percent volume for my HD 600's.
I forgot to mention that there is no noise whatsoever on my example.
Hope this helps,
 
Rich.