I don't think I'd recommend the DigiZod, it's main strength is it's ability to crank up the bass, which you indicated is not a priority for you.
In the sub-$100, probably the cmoy or the pa2v2. In the $100-150 the O2 is definitely an option and people speak highly of it, but it's more "transportable" than "portable". Like, you wouldn't *use* it on the go, but you could take it from home to the office, and back, etc. Not something you'd pull out on a walk or on the bus and start using, though.
I've personally used the iBasso D-Zero for a while with my 4G iPod Touch and HD25-I II's and it is very good. I don't listen to speed metal per se but lots of metal and hard rock (along with other things) and the combo is very good. Maybe slightly learn in the mid's. I just got my C421 (with OPA2227 opamp) delieved a couple of days ago so I'm still burning it in but it sounds pretty impressive so far. It might be out of your price range but it does sound like it will be a bit nicer than the D-zero. Both are very portable and easy to lug around or use on the go.
Let me ask you this, however: Have you tried just running the HD25's out of the iPod directly? If you haven't, try that before spending $'s on an amp. They are actually fairly easy to drive despite the 70ohm impedence (high sensitivity of 120 probably helps) and actually sound quite decent just driven off the 4G iPod Touch. It's worth a try. Let them run for 50-100 hours or so before you make your final verdict, as the headphones get sounding better and better during that time (that was my experience, anyway).
Keep us posted. :)