Welcome to to the club of disappointed Little Dot MK III users. I owned one and sold it after some months.
It didn't impress me at all. I also purchased a pair of mullard drivers (the same ones you got). Marginal improvement, but nothing spectacular. My recommendation: don't waste more money on it if you're not happy: a new DAC won't do miracles. I used Beyerdynamic DT880 with it. I initially tried:
Marantz CD6000OSE -> little dot -> Dt880.
It didn't sound significantly better than doing:
Marantz CD6000OSE (headphone jack) -> little dot.
I then bought a DAC and streamed flac files through it:
FLAC -> foobar2000/wasapi -> HRT music streamer II -> Little dot ->dt880
and compared what I heard with what I got replacing the little dot with my speaker integrated amplifier:
FLAC -> foobar2000/wasapi -> HRT music streamer II -> Integrated Amp -> dt880
This time the configuration *without* the little dot was significantly better. The only advantage I could find is that the LD was completely silent with no music playing (the jack of the integrated produces a faint hiss).
In the end, having decided the LD was worth neither the price I paid nor the additional connections and room taken I sold it. Won't be buying other dedicated headphone amps for the time being.
IMHO: the LD is good if you DON'T own separates hi-fi equipment to start with: it's good for people that don't own, or don't want to own, separates CD players (that have at times excellent headphones amp built-in) or receivers/amplifiers. Most of the ecstatic comments come, I believe, from people that have no non-dedicated amplifiers to compare the LD with.
It's a decent amplifier in a relative sense (relative to powering your headphones out of a portable head-amp or the jack of a laptop, etc);
it's a poor amplifier in an absolute sense (compared to other generic, non-headphone-centred hi-fi amplifiers).
My 2c.