I can't speak to the synergy between the HD-280 and the Icon Mobile but I do use my Icon Mobile with my triple fi 10's. The same thing happens with me as does you. When amped, the sound just has significantly less bass. However, in my experience, the highs are much better with the icon mobile. It really makes the highs on the triple fi 10's really shine. When I go straight out of my iPod classic the bass comes back and sounds much fuller. In my case, it's a decision between better highs or better lows. I'll always take better lows. (Basshead) :) That's why I usually don't use my triple fi 10's with my icon mobile anymore. It really just kills the low end.
I think what it boils down to is that the Icon Mobile's sound signature is that of a brighter, colder sounding amp. It seems that way to me anyway. I haven't paired it with many different pairs of phones so I really can't say what it would do with other phones. If I had to guess, it will probably cut lows and boost highs on any phones you use it with. I'm thinking of getting some Ultrasone HFI-580's. Apparently they are pretty bassy. I'll have to try the Icon Mobile out with them. I think the bass would still be plenty strong enough even with the Icon Mobile on. It might even tighten up. I'm just guessing.
In terms of needing an amp at ALL for your HD-280. I think just about anything improves with an amp in some form or another and given that the impedance on those is 64 ohms, I'd say that the iPod classic just isn't quite capable of pushing them to the limit. Also, I feel that being able to bypass the internal amp with LOD using any amp should be an improvement in SQ. Overall it is in my experience, but as I noted earlier, the Icon Mobile just kills the lows on the triple fi 10's to the point where I'd just soon go straight out of the iPod. You'll just have to keep looking until you find an amp that pairs better with those phones and provides the sound you're looking for. I'm trying to do the same thing.
