Try doing searches on the O2 amp. 0 Ohm output impedance, transparent, and very powerful. Can be built or bought. Built it's like around $140. Can also be used portably. Impedance explained in articles below:
I think this hobby is ultimately more about your preferences than it is about more expensive stuff being better. Always listen to stuff when you can. Most sources and amps do not have verified performance and measurements. People largely determine how good they are by how much they like the components' sound with their own gear as opposed to data that shows high performance in objectively measurable areas. A headphone will sound way different from source/amp to source/amp. It's very difficult to figure out what sources and amps to buy if you're looking to use them as an EQ substitute. You could spend $600 on an amp that may cut off harsh highs of certain phones but has bad measurements in distortion. Or worse. There's been instances where amps were downright dangerous to use because of screwups in the design. Phones are different because some aspects are not measurable unlike say frequency response. Look up reviews of headphones that cost thousands of dollars. None of those phones appears to be anywhere near perfect. I think $100-$200 is probably the last price range before you hit diminishing returns. If sources and amps were measured and people had a chance to hear stuff first, I think this would be a very different hobby. There's a lot of great stuff that's very affordable, so I still buy phones here and there. I think I'm about done with sources and amps as using the more neutral stuff I have now works for me.
For $600, you could get a lot.
2 or 3 of my fav phones. SR-60 with flat pads(at toddthevinyljunkie), K701, K240, Yamaha Pro 400, K550, Amperior
Sennheiser hd280 sounded good to me on best buy's demo wall. Some Samson phones are good too, like the 850.