Woo Audio 3 v Woo Audio 6

I would think with the lower impedance and easy to drive (relatively) Koss cans, the Woo 6 would be the better choice.

Spot on. The A250 are 60 ohm - they will be much better suited for the WA6.

Horay!

 

I was technically correct.... the best kind of correct.

 

I know so cus Rob said so!

This topic has been covered extensively in other threads, and it takes a bit to explain it. The short version is that the WA3 is output transformer less, and as such will have a high-ish output impedance. This will result in a less than ideal electrical match with a low impedance headphone. The WA6 is transformer coupled, and as such has a lower output impedance as the transformer dictates the impedance, rather than the output tubes themselves. This makes it a better electrical match for low impedance headphones.

You can tube roll quite a bit with a WA6 to get a sound you like. I use a WA6 almost every day and really enjoy it.

To supplement Skylab's comment, see post #9 in this thread: http://www.head-fi.org/forum/thread/388373/can-someone-explain-what-otl-means

I can echo the tube rolling bit of Skylab's post. I started with the stock tubes on the WA6, but immediately went to a Sophia Princess and it drastically warmed up my sound. 

IMO Sophia Princess actually isn't the tube to go after if your primary concern is warmth.  It does add a bit of bass extension but also tend to be quite bright and the main benefit is soundstage from my experience.  I'd think one would have much better luck with a 5AR4/CV34 or something like EML 5U4G if you're looking to warm it up slightly.  Driver tubes play a big role as well of course.