This is a tough decision, and one that gets debated quite a bit here. It seems that the differences between Woo products is not very well defined, and even sometimes Jacks responses can be a bit cryptic.
When I was shopping, I had K701's and was debating between the WA2 and WA6SE. The general feedback from Jack is that they are both excellent amps, and will suit different taste. The WA2 is a warmer amp with smoother tube-like sound, while the WA6SE is a faster amp with a more dynamic response and adding a Sophia Princess will add a touch of warmth. He also stated a few times that his personal favorite for the K701 was the WA6SE as he liked the sound characteristics.. for his listening style. He stressed that its a personal thing. This surprised me as I thought the K701 benefitted from a warm amp.
Given that I always thought the K701 was a bit harsh and clinical when not amped properly, I decided to go with the warmth of the WA2... but then a deal came up on a used WA6SE that I couldn't pass up and I jumped on it. I've never looked back. The WA6SE is a terrific amp and sounds amazing with any phone I've paired with it. I think it offers enough speed, touch or warmth, and soundstage, to let the phones do their thing. Power is not an issue, I rarely get past 10-11:00. Sound is open, fast, punchy, and yet has a smoothness when called for. I had a T1, and sold them, I now use a Grado GS1000 and DT990. So ultimately, i don't think you can make the wrong choice between the WA2 and WA6SE, just let your musical preference guide you. Warmth, smooth, tubey get WA2. Fast, puchy, power and soundstage get WA6SE. Then fine tune with tube rolling.