Use "regular" amp to power 'phones?

Is it OK to use a "regular" power amp to power headphones? If so, how? If not, why?

I realize that most amps are expecting to see a load around 4 to 8 ohms and most headphones would present a load of around 600 ohms. I also realize it would be silly to connect a Crown Macro Reference to headphones (perhaps to anything), but I'd explore small tube amps, perhaps SET.
I did this many years ago, I used to do alot of live sound recordings. The only way I could hear anything I was recording was to use a crown DC-75 to drive my headphone's. At the time I used a pair of altec full cup headphones. Sounded really good and I could get it as loud as I needed to. I just had them wired to a 1/4" jack in my effect's rack.
Why not try the preamp outs? I just wired up my new Senn HD600s to the RCA outs of my Aleph P, and it's great! Why bother with another amp stage in the chain? Good luck.
I do the same with my sennheiser 580s. sounds great. as long as you don't want it super loud.
Buy a used Melos preamp. They make great headphone amps. If you don't want to take your chances with build quality, Melos Restoration can service, rebuild etc for you.