What's your OTL tube amp experience and suggestion?

Yes- apparently we're on the same page here, if you'll pardon the expression :)

Simple fact is Most OTL on the market are not built Like Atma-Sphere. These companies that try to work on OTL cant seem to figure out a Stable build, Atma did this decades ago.

The Simple answer to OP issues is, If you want OTL Stick to Atma-Sphere and you will have Years of trouble free operation.

Sorry but DartZeel is not on the same league as the Atma-Sphere Amplifiers. The Atma is a lot more transparent, faster and throws a larger sound stage and does not sound like a tube nor a solid state. Should I even mention the Atma not only has better transparency it is a lot more emotional to listen too. It draws you into the music and does not sound mechanical.

DartZeel make good solid state amp but I'm sorry it cant be compared to a Atma-Sphere.

Tenor has a bad reputation in Asia. They also gave OTL in general a bad reputation. Most buyers don’t understand their were tons of companies coming out with OTL and only to go out of business as none of them could figure out how to make a Stable OTL Design.

Everytime i speak of OTL to someone, Tenor would come up into the conversation as something that used to blow up like a dynamite!!!

Given how dynamic and lively a good OTL can sound, the world would be full of models being offered, if they didn't have their problems.  Most of the ones I've encountered were not fully reliable.  The notable exception are the Atmasphere models which don't seem to die or destroy speakers.  But, I have also heard a number of setups with Atmasphere OTLs and high efficiency speakers where a hum or buzz could not be ameliorated.  

My personal preference in OTL sound, from commercially available amps, was the big Joule OTL (warmer sound than the Atmasphere or Berning), but, the model I heard had problems with tube sockets going bad which caused it to intermittently put out a HUGE pop; a friend with 105 db/w speakers said the pop was as loud as discharging a shotgun indoors.  I heard that pop on far less efficient speakers and almost had a heart attack.

I did not understand the appeal of the Tenor at all.  In the systems I heard the Tenor in, it did not deliver the incredible dynamics and liveliness that is the primary reason one goes for OTLs.

I also liked the sound of the SAP OTL, but, that amp too, turned out to be unreliable.  

The only practical alternatives out there that I know of are Atmasphere and Berning models.  Both are certainly worth an audition.  I am a tube fan, but, I don't like most high powered tube amps.  If I had speakers that demanded more than 40 watts, I would certainly be looking into OTL amps.

I found that the Atma-Sphere Amplifiers sound warmer if you change out all the 6SN7 Tubes provided by Ralph.

The one Ralph gives with the amplifier are Chinese made tubes which to me sound a little bright in the upper regions. Once I swapped these out for good pair of NOS tubes everything changed.

The Noise is most likely from the Tube not the Amplifier. I had the same issue with one channel constantly buzzing, These tubes I used in other pre-amps were dead quiet but in Atma-Sphere they seems very noisy. Once i changed them out for other NOS tubes the noise was gone.

The noise is not from the Pre-amp or the amplifier but the Tubes them selfs, Some of these tubes never show signs of noise in my other pre-amps but for some reason they do in Atma-Sphere.

So the goal is to feed these babies good quality low noise tubes. Once you got the tube noise down and the system dialled in with the tonal balance the system can have a nice warm balancing sound.

Just an advise for those who want a warmer sounding OTL on Atma-Sphere. Use some of the RCA, Sylvinia, Ken-rads. The Sophia Electra used on the pre-amp softened up the system too much to my liking.