I've heard that the Zana Deux has an extraordinary synergy with the Sennheisers. I've been debating with myself which amp to eventually get, whether it's the Balancing Act, Zana Deux, or WA6 SE. At this current point in time, I'm leaning towards the Zana Deux. I believe the WA6 SE and Zana Deux are on the more dynamic side of things while the WA2 has a more lush sound. The WA2 can serve as a preamp I believe, which the Zana Deux does as well. For Senns, I'd probably just save up and get the Zana.
Balancing Act:
$4000
Fairly expensive and I would have to get all my headphones rewired to take advantage of the balanced aspect. I do believe it has an SE output however. I'd also have to swing for a balanced source as well. Balanced is a step above Single End, but whether it's worth the hassle and cost of building a rig around it is another story.
Zana Deux
$2500
I can get it and the HD800s for the cost of the Balancing Act and without having to get anything rewired. An 'end game' solution for the single-ended side of things.
Woo Audio 6 SE w/ Sophia Princess Rectifier
$1200
I believe this is a lot of 'bang for the buck.' Has dual inputs so you can listen to a high impedence and a low impedence phone at the same time.
Woo Audio 2
$950
Haven't done a whole lot of research on this amp other than hearing it's differences in sound with the Woo Audio 6. I believe it ultimately depends on the type of sound you're looking for.
http://www.head-fi.org/forums/f5/woo-audio-2-a-402215/
Me personally, the more I think about it, I think I'll swing for a Zana Deux. Was thinking about the Millett 307A at one point, but at $6,000, it's ridiculously expensive. The Balancing Act is a more reasonable $4,000, but I'd have to build a balanced rig around that, which is expensive.
Woo Audio 2 or 6 SE vs. Zana Deux
I think those two are more bang for the buck, but you might get that audiophile itch in you and start to 'wonder what you're missing' when people say the Sennheisers sound so awesome with the Zana. Most likely it'd be best just to save up and swing for the Zana so you won't have too many thoughts lingering in the back of your head.
Hi-end balanced is a whole different price bracket, so that is out of the equation for me for a long while.