headphone question

I'm going to buy a new pair of headphones for occasional listening, such as the AKG K142HD or K272HD model. Will I need a headphone amp for these phones or will my McIntosh C712 pre-amp be sufficient since it has a headphone jack? Any other suggestions would be helpful.

I don't think so.

I do think having one though, likely may improve on the sound you will get... as with about everthing else, dedicated upscale items should increase performance.

The impedance of the phones you want to get will give a clue there. Higher imp's phones might need one. A quick message off to the company will definitely tell you... or to one of their dealers.
I recommend you give the 'phones a try using the Mac's built-in headphone amp. If the sound doesn't suit you, then go buy a nice amp...