So I've been able to gather that there are basically two ways I could go from the responses so far (and thank you to everyone that is contributing, it really does help):
1) Buy a set of Sennheiser HD6xx and as much headphone amp as I can get with what's left over.
2) Buy as much headphone amp/dac as I can get my hands on for $300 (no headphone purchase).
I know that I don't have incredible cans so far, although my EarTech triple driver custom IEMs are stellar, and I've heard them at EarTech's place through a headphone amp and know it will help them to shine. I have a tiny FiiO E6 amp and all of my earphones sound a little better through it, though it might be a placebo effect- in which case I guess I'm easily fooled. So the thought of a really nice headphone amp intrigues me. But I'm also a guy who loves earphones and the thought of a better set of earphones is exciting as well.
I apologize for not being more specific until now, but I've really just had time to think about all the responses and am next to my gear to give more detail:
The vast majority of the time I'm plugging my headphones into my Yamaha CR-800 receiver, which is what I have ALL my speakers going through right now (a pair of Cerwin-Vega AT-15's and a pair of JBL SLH610's) and I've got my Kenwood KD-2055 playing through the Yamaha. Additionally, I've got my Mac Mini and PS4 connected via HDMI to a Sony PS3 3D monitor and am going out through the headphone out of the screen into an RCA splitter into the Yamaha receiver
OK,quick vintage receiver 101 lesson...
Vintage receivers like your Yamaha are great for high ohm(250+)headphones.Back in the 70s there were no earbuds,or mobile devices so most headphones were of the 250-4000ohm variety.Receivers back then were designed to drive these beasts.If you look at my signature you will see all the vintage receivers that I own.All drive planars and high ohm headphones very well.
I also own a proper HP tube amp and on some of my headphones(HD250 300ohm)prefer the tubes.Others like my AKG K240 sextetts(600ohms and far less sensitive than the HD6xx)prefer the vintage receivers.
What you want to avoid are headphones like your Grados into the Yamaha...impedance mismatching can cause bass bloat and overall loss of clarity.
If I were you I'd invest in a high ohm headphone like HD6xx,HD600,HD650 or a planar like HE-500,HE-560 or HE-400(really any other than the Abyss or HE-6)All of those will sound great from your Yamaha.
For more info you can join us that love our vintage receivers as headphone amps in
this thread
This ^