The Zana is fast and has amazing bandwidth out to 60kHz. It is crisp, almost like solid state, but just smooth enough and with a touch of warmth that only tubes have. It has terrific control over the low end, as well, which isn't always common with tubes. It has worked well with every headphone I have, save the K-1000. It just doesn't have the power to make a K-1000 sing. Mostly, I run the HD-800 off the Zana, but have also used a variety of Grado, Sennheiser, AKG, and Beyerdynamic phones with it. IIRC, the Zana has an output impedance of around 12 Ohms (great for an OTL) so it does well with low-impedance headphones and has a terrific damping factor with Senns - probably why the HD-800 gets along so well.
The other amp I keep in rotation is a Moth Si2A3. It sounds more like the stereotypical tube amp - it has DHTs on output transformers. There is that magical DHT midrange that almost sounds better than real life, too. Also, the Moth uses no feedback while the Zana uses a small amount.
For critical listening, the Zana is terrific. I also use it for relaxed/pleasure listening, especially for rock and when I want tight lows. The Si2A3 is truly special for vocals and acoustic music. Like I said, the DHTs add something that might not exist in real life. Normally, I hate that sort of thing in audio gear. But it just sounds so damned good that I can't help but use it. Also, the Si2A3 drives the K-1000 adequately off it's speaker taps. That's a glorious combination for acoustics and vocals.
