when i got my bifrost it was a demo, already had some hours on it, so not sure how big a difference the burn in made
now when i had a modi, i found it a little harsh , after a couple of days, much better, no harm leaving it on overnight.
as for the little dot, tubes can change significantly with some hours on them, but don't leave it on for more than 6-8 hrs
one of the great things about the little dot is tube rolling is pretty cheap
what headphones are you using ??
I'm using the Shure SRH 840's.
My brother is about to move the K1/K2 switchers over to the 6J1 (left) side.
Hopefully this will improve sound quality, we forgot to move them.