hhhm I hear the HD600 have a kind of boring sound signature, I'm after some low bass punch and exciting / bright mids and highs, wound't a Beyerdynamic 'something' be better ?
I think I've got some good advice for ya but I gotta warn you that it is subjective with no dbt tests done. Anyway, I have listened to the ATH M50s. Also I have used the HD600's for over a decade. First they are right, it's not your amp that's going to be the biggest change. It's your headphones. The ATH does sound nice and reminds me a lot of the HD600s but with the right amp the HD600's sound superior to me. However, it's not a BIG upgrade.
If you are looking for a punch and slightly brighter sound with more detail, you could use my current setup which is the Sennheiser HD700. It is punchier, extends deeper and the soundstage is somewhat bigger and feels slightly outside of the head. Also if you crank it, it's a real gas. It's got the capability to swing bigger dynamics cleanly. I never realized my HD600's weren't capable of dynamics as it's pretty great, but compared to these, the hd700's stand out on dynamics. . The sound is a little brighter and feels very detailed.
It's definitely got bass. If you want more, the no-tools, completely reversible modhouse felt mod for $15 for the HD700 cranked up the bass more It has two parts - a felt ring, and thick add on pads. The add on pads give you more bass but it's not as bright. So you can check out the three options - stock, felt ring, felt ring + pad you like. All three are quite nice and they sound like different headphones.
So that's my suggestion. There's always better headphones, But in your price range, these are pretty nice imo and may deliver the sound you are looking for. Brighter, punchier, more dynamics, larger soundstage.