Which amp tube, input or output?

Yes you will.

Yes, you should. I use different input, output and driver tubes in my Audion Silver Night to different effect. The driver makes the least difference in this amp's design, but I still have couple of favorites. I have only used various versions of the Mullard EL-84's in the past and cannot make any comparisons to the modern versions of this tube. I think that my favorites had metal bases, but I may have them confused with EL-34 tubes that I also used. It's been a long time.