Since you're happy with th eGrado sound,... stick with that, and scale up that ladder.
What sony PCDP is it? many vintage sony players have plenty-good headphone outputs, that are more than adequate power for Grados. IMHO the "mega-bass" feature on the mid-late 90s discman players is a very good sounding bass-EQ. if that discman has a toslink output, that opens up more options for you... if you have a home receiver with an optical-input that can serve as an outboard DAC.
Sell the SR60 for $55, take lots of pics and include any/all packaging. Toss in the 1/4 adapter if you still have it.
That will leave you with ~$330 total. That will easily get you a used MS2+cmoy. You might be able to find a deal on a govibe6, instead of the cmoy. IMHO that will blow away an unamped SR60, and is a worth while investment.
$330 will easily get you an HF1 if you wait around long enough for one on the F/S boards. You might even find a used RS2 for that price.
Vintage Marantz receivers are known to have exceptional headphone amps when paired with Grados. Its not uncommon to find them under $100. IMHO thats worth a look too.
I'd try and find a used RS2, and just plut it into your discman and rock out. Either that or an MS2 + amp and use the line out on your discman to source it.
Good Luck, you have a lot of opportunity with that kind of $$$ to improve on an unamped SR60.