How is the OTL sound like?

I use Atma-Sphere, the M-60 Mk II.2. These are a revelation. Their reproduction of dynamics, high frequencies, timbre, timing, bass, control is amazing. These are easily the most fluid, grain-free tube amps I ever heard. Super fast, clean and clear, natural and balanced for the whole spectrum, they are superb. I believe, I had a mesmerized look in my face, when I first listened to them. There is everything I hear from my Pass amps AND MORE. They give me every detail with lots of air, combined with brutal power in the bottom without being euphonic linked with stunning speed and transparency. In any case, the sound these amps produce is almost frighteningly real - clean, open, "air", wide soundstage, unbelievable detail - it's all there and then some. And : there is NO "warmth" or other tube-related anomalies - just clean, accurate musical amplification.

Thomasheisig is spot on. OTL's are simply more pure and natural but need to be matched with the right speakers to get the best out of them, and extend their tube life. Sensitivity is less critical than the ohm load they produce over the frequency spectrum. Look for speakers of at least 6-8 ohms with minimal phase angles. The higher the Ohms, the more dynamic and powerful will your amps sound.

Ditto to what's been posted above. When I first heard my Atma-Sphere M-60's (I bought them used, without an audition) I literally had that same warm/fuzzy feeling I had when I was courting my wife.

Thomas, did you first hear the M-60's paired with the Köchel's? If so, I can imagine how intensely profound your experience was. It is profound even when listening to this combination on a daily basis.