I'm a huge fan of their amps. All of them.
My first portable amp I ever owned was the Total Airhead back in 2003 or 2002 and I was kind of clueless back then (No head-fi!). I think at the time I was using the DT-990 and an AKG K701 with the Airhead
I came to the conclusion that yes, the K701 is not a good match!! Gotta love the red blink of death telling me I'm a moron every second. I loved that thing so much that when I accidentally blew it up last year, I immediately bought the same thing.
They're $99 and you can get cheaper portable amps made in China, but all their amps seem like they're built like a tank. I believe they're made in the USA too. You know the Micro Amp that looks like all cheap plastic on their webpage? It's not, but made out of metal and I dropped it a total of 4 times. How is anyone dumb enough to drop an amp four times? Well, it's still alive.
Back years ago Headroom amps were always mentioned a lot more on the forums. Now it seems like very few people mention or use them.
The Micro Amp is my favorite amp I've owned and has driven everything extremely well. HD-650, HD-600, K702 and even the K501. The only negative is that I get lots of clueless people telling me it's a portable amp and can only low impendence headphones. It's nearly half the size and feels like an E9 on steroids. Only headphone it didn't sound well with is the AKG K240 Sextett. It's not because it didn't have enough power. I actually prefer the Micro Amp with the HD-650 to the Asgard. I bought the Micro Amp (and kept it) due to playing well with both my HD-650 AND K702. The Asgard didn't get it done, but is still a very good amp.
I really hope they come up with a new portable amp smaller than the Total Airhead. Maybe something USB chargable. Total Airhead is nice, but it's a bit bulky. That thing can even drive the HD-650 quite well though! With the newer HD-650 the red (over voltage) light almost never went off even at very loud volume levels. Can't drive the DT-880 600 ohm though