I have fostex-t50rp, fostex-t60rp, sennheiser-hd600, hifiman-he400s, and stax-sr-l300. Honestly, I least like the sound of hd600. For me, he400 is even better, although some people showed that both hd600 and he400 have the same sound characteristic, flat and neutral. Before owning t60rp, t50rp provides the most enjoyable sound. In term of cost effective, I would suggest t50rp.
If you want to go with hd600, I also suggest to change the cable. The stock cable is suck.
I use fostex-hp-a4 bl and darkvoice. However, there are many high power amp below $200.
To my ears, t50rp is more sensitive to noise than hd600, so you may consider a good power cables and noise cancelators.