Are Headphone amps worth the expense?

If it sounds better than what your currently hearing, than yes. Otherwise no.

Sorry for the smart-aleck answer, but as with all things audio, any component, even expensive ones, can be better or worse than whatever you currently have. My headphone systems sound much better than most speaker systems I've heard, but I've spent many years getting my headphone systems to the level they are at. And to me, a tubed headphone amp is indispensible.

Oh, and you'd do better to look on head-fi for headphone info.

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.