What are the basics of headphone amps?

I have a Krell KSL preamp and KSA-150 that have given much pleasure over the years. I now am at the point where my listening needs to be more private; I need headphones.

A number of emails to Krell have resulted in the advice, "get a headphone amp". I have pressed for more details but silence has been returned (sometimes we like silence but not in this case!).

It seems from my Google searches that there are many headphone amps for doing mixing and for supporting a computer or a travel CD. I can not find a single headphone amp that seems to function as an interface with a high-end preamp.

What am I missing? Can someone fill in the blanks that Krell has so poorly left?

Why can I not just get two RCA to phone adaptors and plug headphones (yet to be acquired) into the KSL outputs? How about using the Tape-Out outputs? Does the KSL not put out enough power to run headphones?

Thanks for any help you can give, James
lots of info on these sites
Thanks, Tom. I'll take a look at these sites. Much appreciated, James
Cayin, Ming-Da, and mant other Chinese manufacturers make tube headphone amps that have one or multiple jacks/outputs and volume control.
Although headphones obviously don't require much power, they typically have an impedance in the range of 32 to 100 ohms. That is much too low to be driven properly by a line-level or preamp-level source.

There are lots of suitable amplifier choices available, including the tube-based units Cajunpepe mentioned (Antique Sound Labs is another such manufacturer), and many solid state units.

Some selections to consider can be found here.

You may also want to consider pro-oriented headphone amplifiers, many of which can be found at bhphotovideo.com (enter "headphone amplifier" into their search feature).

-- Al
If you don't find any here,try Audio Advisor.They have a good selection.