I confess i have not used it as a pre-amp simply as a headphone amplifier.
What attracted me to The Little Dot was :-
Good internet following and reviews.
Good price value - i bought when £1 was worth $2 so it seemed particularly cheap back then for me, but is still good now against mainstream products.
Good support - the people behind this offer quick reply to any queries and generally have a good report for this - they even seem to replace items without fuss if needed. The downside is returns would require overseas postage. When i raised some questions before i bought one i got replies within 24 hours each time.
Great looks - i mean have you even seen anything much like the valve based ones in the shops. There is also a non valve solid state version V too if you prefer this style.
After receiving it and running it in for c20 hours to get the valves going it really does sound amazing. I have the Mark III which is a valve one and it has a really open soundstage with my Sennheiser HD595 phones. There is no background hiss or noise polution. The musicality is excellent and is better than my Arcam A32 FMJ socket (which i have to add is itself way way better than other separates headphone sockets i had tried before including Nad, Cambridge Audio and Roksan Kandy).
I would guess it has a slightly warm and vibrant sound, and is great with female vocals especially but ive not been dissappointed with any music (i dont really listen to heavy rock or rap). Its hard to describe in comparative terms as i not heard many other proper headphone amps but i am very happy as it is a unique item really. Also you can change the valves off ebay to different models and this can deliver a different sound empasis. I have some Russian ones which do add a touch more bass emphasis, but the originals are fine too.
For me its a great value item, sounds lovely and has a fun factor too. Listen in the dark with the valves glowing - great stuff.