The Naim Headline barely gets any attention around here, but if it does, it's positive.
I auditioned it with the HD800 some time ago, but found my Stax SRM-T1 / SR-Lambda Signature combo better. Unfortunately, I do not have experience with much other dynamic cans and amps, so I can't help you with a comparison. I did not have the feeling that the HD800 was underpowered by the Headline, but again, I do not have much experience with dynamic cans.
As a power supply I used a TeddyCap. Much cheaper than Naim's power supplies and (supposedly) as good as or better than a HiCap. At least it was a real good upgrade to my CD5X. You should look into it if you want to upgrade a Naim product with a power supply.
I think a problem of the Headline is that it gets expensive really fast, because you have to buy a good power supply for it and Naim charges a lot of money for that. That in turn means that the price/performance ratio is probably less than the other amps you mention. Third-party power supplies could fix that.
Bottom line: I think the Headline suffers from good competition, little exposure (around here) and the power supply issue.