Are Headphone amps worth the expense?

I would get a headphone amp on trial for comparison. Grados are easy to drive and it's possible you will not hear a worth-your-money difference. Your dealer should be able to help you with hook up.

Are you into DIY?

Try building a CMOY headphone amp. Strictly low cost and fun. The good news is you can than 'roll opamps' and test the different ones available.

I use OPA2134s as a good compromise.

Very good amp when properly built. Not as good as the 'best' but no more than 5% of the cost.

http://tangentsoft.net/audio/cmoy-tutorial/

Headphone amp? I assume your MAC has a phones jack?
If so.. you would have to spend around $1000. to get a headphone amp you will notice is really better.
The ones that cost less will sound 'different' than your Mac, but not realy 'better'.
I own Beyerdynamic old 990's and Sennheiser new HD800 phones along with:
The jack in my Bryston BP-26
the jack in my Sony 555ES changer
The headphone amp "Lil Dot ($250)
the TADAC DA converter with 'tubyness control' and headphone out (used $500) How could anyone resist a 'tubyness' knob?
the headphone amp Rudistor MkII ($1,800)
The headphone amp SinglePower MPX with major upgrades ((Retail with extra tubes and adaptors $4,000==== I paid $1,000 used for my 'exploding killer deathstar' headphone amp. (LOL inside joke))
The Bryston is good. The SinglePower with 6BL7 tubes is the best ... dangers of sudden death due to use notwithstanding (many strange and crazy posts about SinglePower being dangerous and going blind and being electrocuted or burning up house and home over at HeadFi made it irresistable as a conversation piece, and it IS a good headphone amp.)
Anyway, unless you can try one, Or want to spend a lot.. I would say you are just as well off with your MAc phone out.

Some specialized headphone amplifiers simulate crosstalk between channels making sound appear in front and not inside of the head. It cross-feeds portion of delayed sound to the other ear - the same way that it happens when listening to speakers. I had sound card with this option and it was great. It is also possible in analog domain - I have link to Headphone Amp project with cross-feed somewhere. I remember seeing headphones that have built in crosstalk (tube) from one cup to another - but don't remember the name.

I found the link: http://gilmore2.chem.northwestern.edu/projects/meier_prj.htm

That's what I didn't like about phones...it always seemed like the sound was coming from the sides or actually behind you instead of in front of you the way speakers sound. Maybe I need to research amps and find a decent one with crosstalk. Anybody have suggestions?