OTL is a bit of a general term and results can differ widely depending on the circuit implementation quality of power supply and of components and tubes used.
True.
Though I think the 650/6xx work best with OTL amps is born out of the fact most OTL amps have high output impedance suggesting they need high impedance headphones. And of course, 650's are 300 ohms.
In reality, many headphones show no difference in frequency response regardless of amplifier output impedance.
The Feliks Elise is a good amp and in fact one of the amplifiers many suggest does lift the Sennheiser veil. I've tried the 6xx with many different valves on an Elise and they all sound good, but there is no lifting of the so-called veil ime.
I think impedance mismatching is over stated, sure it does happen now and again. Similarly, I'm currently using the Feliks Elise as a pre-amp into the power section of a Marantz PM8006.
Bearing in mind the oft-quoted advice is for the power amps input impedance to be 20x that of the pre's output impedance...
- The Elise is 2.5kohm output
- The Marantz is 15kohm input
So only 6x greater. Sounds fine to me.
To test I hooked up a Schiit Loki in between the two. This has an input of 47kohms and an output of 75ohms, meaning the Schiit's input is close to 20x that of the Elise's output and the Marantz's input is now greater than 20x that of the Schiit's output. Effectively an impedance buffer and eq.
And... it makes no difference to my ears, switched on or off, or not even in the chain. Obviously, if I start eq'ing with the Loki it makes a difference.