Well I like that suggestion since I'm likely to build a Bottlehead crack if I do get a new amp. I figure if I don't like it I can always sell it for a slight profit if I don't mangle the build ;)
I'd say one thing I'd love to get out of a new amp is more detail at a lower volume. When I really crank it up, and yes it does get louder than listenable, its like dunking my head in an ocean of sound. The sound tends to get bigger and more immersive when I turn the volume up on the hd-650's, not just louder. It can truly be a sublime experience, but I have to turn it up to the point that I can't possibly focus on anything else at the same time. My headphones play through my computer and at the very least I'm usually studying, doing emails, or browsing forums while listening.
Aah, this makes things clearer.
To start, it is against the rules of the forum to sell DIY gear at a profit lest you be marked as a MOT and have endless restrictions placed on what you can say. The fact that these rules are in place doesn't stop anyone from trying, but it is much harder than it sounds.
Building a well documented amp like the crack is a great way to start out with DIY stuff (although the voltages are on the high side, so keep that in mind) and at the worst you shuould be able to sell it at a small loss. I just would not go into it planning to make a profit, it only very rarely works out that way.
On the headphone front:
If that is the sound your looking for (I like a similar sound - good detail at low volumes) I'd try other headphones. The Senn HD580/600 are quite nice for this on a budget. I would lean towards 580 because they are less expensive and I prefer the looks, but they sound the same. Neither one will be hard to resell if you decide you dont like them.
