Question for tube aficionados

Amperex Nos or decent testing units 12AX7 
orange globe 

Thanks to everyone for their suggestions.

My speakers are modified Dalquist DQ 20’s.

Overall, the system is fairly well balanced.  What could be improved is occasional brightness in the treble, especially in the line stage.

 Ideally, I’d like to replace all six tubes, depending on the cost.

Vintage pre-1970 GE, RCA, Sylvania, Mullard and Telefunken are still the best made and sounding of the 12a*7 types! Far better (and longer lasting) than present day production! That is why they command high prices!

Cheap present day 12a*7 types will never equal the sound quality and lifespan of vintage types!

I have always gotten solid advice and feedback from Brent Jesse, Andy at Vintage Tube Services, and Upscale Audio. I’ve purchased from all three over the past 20 years, and have not gotten a bad tube from any of the three. 

I would start by replacing the two 12AU7s in the line stage with a very high quality matched pair, as all sources will run through that preamp stage. Replace each additional tube type in phono section in stages to evaluate what each is doing to your sound for your future reference. You may notice some tube positions make larger differences than others.  Have fun!