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.

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 ,