Preamp tube

How critical are the 12AT7/ECC81 to change?

See your other thread. They will create a modest change in sonics.

Definitely swap out the Genelex for some NOS tubes. Choose a tube based on how you want to change the sound; eg, warm, neutral, more detail, larger soundstage.
Here is great info on tube brands and their sonic characteristics...

http://www.audiotubes.com/12ax7.htm

I think I may have posted the same question twice on AG, sport about that. I’m getting great responses from both and I appreciate your input. How critical are the 12AT7/ECC81 to change? Is there a sonic benefit? The most I’m hearing are the AX7’s play the most critical part in voicing. WhT do you guys think?

 I use NOS Telefunkuns from the 1960's.  They are expensive but were a clear improvement over my existing tubes. 

Ampex or RCA .Pulls from Baldwin home organs are usually good  as well ,

Sylvania-Baldwin green label circa 1960’s is a nice sounding 12AX7. They can be had on ebay at reasonable prices!

Why not try some NOS? American and European tube companies in the older days of audio  made great vacuum tubes. Sylvania, RCA, GE, Mullard, Raytheon and many others...