There  WAS a version of the 6AC7 that was variable mu. Something like 1851, but there were very few made, and I haven't seen one for sale for a very long time.

Although I am not certain about this, I believe that the most critical parameter in receiving tubes that determines their "quietness" and gain is transconductance.

Also, generally, the more grids, the more noise.

Ken W7EKB