Sexy yes, buy silly, neigh, stupid, in my opinion. For many reasons. Amps may tend to "sound different", but they really shouldn't (especially when by different, we are really talking about poor implementation and/or distortion). If you want to control the sound of any system, I much prefer doing it with new speakers or with 'select' electronic purchases (ie, I get the electronics that I think meet a baseline performance target with a per-determined budget).
The other critical factor (for me) is cost. I just can't spend more on electronics than I do on the phones / speakers themselves. I strive to hit a certain benchmark of features, power, and performance with my electronics, but I have always found that going cheap on your speakers is the dumbest thing you can usually do (with the exception of great values that ARE out there). If you really want a new sound, get new speakers. Or better yet, a new album. That works every time.
Going from Realtek to Soundblaster X-Fi was an improvement in the sense that I gained some sound performance, features, and power over the stock chipset. Later, I decided I like the realtek 'sound' more than the soundblaster, or at least enough that an upgrade wasn't really worth it in terms of getting better headphone performance. The O2 amp is superior to both sonically, and puts out more power than I should realistically need for virtually any headphone I may buy in the future. For $120 bucks, it is clearly a good deal at 30% the cost of the HE-400s themselves. If I had got a Lyr, I'd be close to doubling the cost of my headphone setup and I am certain it would not double performance to my ears (even if it was nice). In like fashion, the O2 didn't give me a night and day performance change, per say, but it was a definite improvement. I could spend 50 times more on an amp and I highly, highly doubt the HE-400 will sound much different, and I guarantee that Type-O Negative CDs will still lack bass slam!
I've said it many times before so I'll try to be brief. DON"T but an amp seeking a 'different sound' - that will lead to disappointment or poverty. DO buy an amp that meets your power requirements at low distortion.
Yes I was only looking at getting an amp as it would be sort of step one of an upgrade... So if I then went to some better headphones than the HE400 I would not need to buy an amp AND headphones at the same time. But TBH you are probably right about most that you say with the HE400 anyway. I like the look of the Audio GD products but it does worry me a bit buying from some chinese vendor etc. I think they need to improve their distribution and marketing.