Airtight 300b or Coincident 300b?

Chaccone, 
There are definitely very high quality 300b SET amplifiers in the marketplace to choose among. I have owned the Coincident Frankenstein MK II for 7 1/2  years and it is wonderfully natural and realistic in its presentation. The Airtight 300b SET has a simply stellar reputation and I believe that you'd love it.  

I could confidentially say that about Zanden,Shindo,Triode Corp of Japan (upper leve) Ocellia Signature  (with the silver wired transformers), Ancient Audio , all of these would be excellent choices.  You will never get a consensus of which is the best. The point is that you'd be thrilled with "any" of the SETs listed here and others examples I'm sure.

Any of these superb 300b SET amplifiers with the proper speaker will just sing and bring genuine musical joy into your home. 
Good luck, 
Charles 

If you're not totally set on one of those two amps, you're obviously an Audio Note fan, how about these:

https://www.audiogon.com/listings/tube-audio-note-uk-vindicator-silver-signature-2017-03-09-amplifie...

Not affiliated just a big AN fan and a big 2A3 fan. Never heard either of the amps you're asking about, though I did run an Air Tight ATC-1/ATM-1 combo for a while.

Yes, all amps employed EML 300B XLS tubes. 

The current Frankenstein.  Thanks for sharing.  for your audition, were both of the amps using the same 300b tube ?

Which Coincident 300B amp are you comparing?  The current Frankenstein or the older MP300B?  I have listened to the Airtight ATM300B and both Coincident 300B amps with Avantgarde Duo speakers and my preference was for the Airtight.  Very quiet amps with a lot of detail and dynamics.  Very well made also.