Are most tube preamps ‘ultra linear’?

One thing That is good to look for is a preamplifier operating in pure class A . I find their purity very appealing 
a  DHT is the purest single ended But it lets you hear everything 
not all speakers and amplifiers are a ideal match . I have owned 
several hand built models from Vacuum tube Guru Radu Tarta 
exceptional build and sound quality.

@audioman58 : All tube preamps operate in Class A! They have a single tube connected to the output straight - or through a cathode follower or a transformer! 

DHT triodes are almost never used as preamp tubes because the gain is way too low - plus noise is a problem! 

Hello Jason and others a DHT preamplifier,
is single ended and the purest type of Vacuum tube preamplifier 
it being Direct Heated Triode has a Sonic purity most people 
Have not had the opportunity to hear. I mentioned RaduTarta 
for he builds these one at a time and prices can vary greatly 
depending on parts quality and transformer and Chokes quality .
something worth investigating if you truly want something exceptional starting maybe around $4k , 

Not all preamps are one tube, single ended. Some use two tube and are push/pull but all are still class A. The low power of a preamp is imple to make class A.

Single ended amps are not so obviously the purest thing around. They are low power with lots of second order harmonic distortion. They react with reactive speaker loads so their output is not flat. And they are non-linear. They compress the negative half of a wave form relative to the positive half.

Every format has trade offs.