One other possibility that comes to mind is the Simaudio Evolution, I did not listen to it over here in the UK but that could be my loss.
I would had suggested Pass Labs, but I know Keith Howard was disappointed in it, due to being highly impressed by the Pass Labs seperate XA power amp.
Just remember to listen to both XLR and RCA on the Dartzeel, makes me wonder how many other amp setups have subtle audio differences between both.
This can be important as I love the Zeel using RCA, which is not as forward/explicit IMO.
Unfortunately being here in the UK I did not consider Boulder.