Just an ironic note : Couldn't the slight "brightness" you mentioned be a problem with the Focal Elex, rather than the LP? I ask, because I have a metrum acoustics ambre and the Metrum Acoustic NOS DAC feeding my Liquid Platinum. The Metrum NOS DACs are generally known for being laidback, and they usually act the part with the Hifiman He-1000, or the Audeze LCD-X, or the Audeze LCDi4 playing on the attached LP : Wide, Airy, and deep sound-stage, black background,, great extension at both ends, and great detail retrieval with the treble smooth as a baby's bum...
A couple of weeks ago, however, I tried my newly acquired Focal Elex on the same rig, and it showed all the listed characteristics, with the exception of the "smooth" top end part. Using the stock balanced cable that came with the Elex showed the top-end harshness and aggressiveness at its worst. Replacing the stock cable with a Norne Draug v3 cable tamed things down and smoothen things out somewhat, but it did not eliminate the harshness entirely.. Finally, I replaced the Draug v3 with a Silvergarde S clear cable, and while it did not entirely eliminate all the aggressiveness, it toned it down enough to make the Elex listenable and even enjoyable... The experience was the same for all tracks I listened to, and not just for badly mastered ones... I was actually listening to a lot of ECM label Jazz albums on Roon/Tidal, and those are among some of the best mastered albums I can think of.
This encounter of the Elex with the LP in the chain I have described, eventually led to my decision to sell off the newly acquired Elex, with a lot of sadness, because I found a lot to like about the it otherwise. The point i am making is that, because all of my other cans have relatively much better synergy with the LP, and behave much better to my ears when playing the same albums on the same rig, I never really saw the the LP as the source of the Elex's harshness and glare. And that is what makes your conclusion interesting to me, as it is making me look at the LP in a different light for the first time.
Of course system synergy and subjective impressions are paramount, so the most reliable conclusion I could draw from my little experiment was that the Elex was either not entirely palatable to my own personal hearing faculties, or it wasn't always playing nice with my rig, and so I do not completely regret deciding to let it go. I thought the story was, however, worth sharing in the light of your own experience.
From your description, it appears your NFB1-AMP shares a similar sonic signature as my Violectric HPA V200, which pairs wonderfully with the Beyer DT-1990. I'm thinking the LP with proper NOS tubes will fill in a slight U shaped curve that these Beyers have, without any increase in treble hardness or grain. Have you rolled any other tubes in the LP other than those Mullards you mentioned, and maybe comment on how they affect the presentation?
