Here is an easy (but relatively expensive) solution (at $99) for those who want to connect single-ended headphones to a speaker amp:
The APPJ Headphone Adapter
Available in black or silver, this photo shows that it has banana plug jacks at the rear, so to connect it to a speaker amp, all you need is four wires, each with banana plugs at both ends.
At the front, it has one 3.5mm TRS jack and two 6.3mm TRS jacks. It's not clear which jack offers which level of gain (or attenutation, actually) but the 3.5mm jack is most likely the low gain jack.
ALO is a reseller: http://www.aloaudio.com/miniwatt-headphone-adapter
UPDATE on 22 January 2013: It seems ALO no longer carries this product. A similar product, called the Miniwatt Headphone Adapter, can currently be seen at the website of ALO's sister company, 32 Ohm Audio.
ALO does not provide specifications at their order page, but here are specs from the DIY Audio link, below:
APPJ is the same company that produced the (2.5W into 8-Ohm) 6F3 Mini Tube Amplifier
Edited by zilch0md - 1/22/13 at 6:33pm