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