Headphones powered by a preamp output?

Has anyone explored this idea? I'm thinking about ways to utilize the 2nd pair of XLR outputs on my preamp. Isn't it possible to wire the 2nd pair of outputs to a headphone amplifier or would you have better results driving a pair of headphones directly?
The XLR outputs will not deliver significant power to the headphones. You'll need a headphone amp.
The Headroom Blockhead can be purchased in a balanced configuration, but you will need specially wired Senn 650s to listen to it. check out head-fi-org.
Thanks for the information. So a headphone amp is the way to go.

I'm interested in a Stax setup that has balanced inputs and uses tubes. Does anyone have any experience with Stax?
I am not sure that you have been given a complete answer.

With Krell preamps, there is plenty of power to drive most headphones. To use the XLR output, you will need an adapter. I am not sure if this is the best way to go, but it is a possibility.

I do it with my BAT VK-31se with pretty good results. You do not get super high volume and the highs are a little rolled off because the phones have a relative low impedance. Depends on your pre. The BAT has a pretty heathly output and has a single ended output as well as xlr. Recommend highest impedance phones (I use hd-600s, 300 ohms). Picked up an RCA to 1/4 stereo adaptor for the connection (radio shack). You problable will not find an XLR to 1/4 inch. You might need to get cardas xlr to RCA adaptors (female XLR to female rca).

Which preamp do you have?

Definitely worth a try.
With a Stax setup, the amp usually comes with the headphones. AKG K1000 comes with XLR cables.