The idea of swapping out tubes is kind of fun too. Someone said they swapped out for these, and the sound quality was very noticeable. Not sure how true that would be. At the very least, it seems like a fun entry-level unit that I can have some fun with.
Glad I could help. I do have my Mk2 hooked up to my 789 through the passthrough (RCA SE PASS outputs) on the back of the 789. All its doing is letting the signal run straight through the 789 from my DAC on my soundcard to the Little Dot Mk2.
You should be able to tell a difference between the Little Dot and the THX 789 if you have everything hooked up correctly.
As for your question on tube rolling, yes, tubes can help with fullness, soundstage clarity, bass, treble etc... its just a matter of finding the right pairings and that Is why I not only listed recommended tube pairings for the LD, but the guide on head-fi that describes in detail and answers many questions you likely have as well.
Get the Novosibirsk tubes and these Russian Rocket "Voskhod" tubes... heres an amazon link and I bought these exact ones and they are the best ones Ive tested. The head-fi guide guy also attests to this and I do personally.
Vokshod Rocket matched pair Amazon:
https://www.amazon.com/Voskhod-6J1P-EV-6ZH1P-EV-Matched-Pair/dp/B071H78DHZ
Novosibirsk matched power tubes on ebay (this is a good seller and they are located in the US with fair prices and good quality control):
https://www.ebay.com/itm/290614193946?hash=item43a9f4af1a:g:81MAAOxywbNQ93EE