Different Tubes and general purpose?


I’m trying to get a better midrange push or clarity out of my Line Magnetic 501ia tube amp.   Can someone help me understand what the specific function each type of tube performs in a power amp?  

I would like to upgrade several or all of my “stock” tubes but would like to understand where to start to beat enhance each aspect of the sound spectrum.   On my 501ia i have 4x KT120 and i believe those are power tubes, but please help clarify if wrong.  There are 2x 6SN7EH 1x 12AU7 and 2x 12AX7 (but the AU7 and AX7 look interchangeable to me).   As i look to upgrade where to i look to start, and why?  
Thanks in advance for help. 
gunners01
Saying "upgrade" does not make it upgrade. Changing tubes is not upgrading anything. Differences between tubes are slight or subtle at best. Someone here will jump on this so have to say yes there are differences. In the big scheme of things though you will hear much more difference changing the fuse inside the amp than changing the tubes.

That is to be taken literally. The difference in midrange clarity and detail from a $160 Synergistic Research Orange Fuse is a lot more than any tube you can find at any price anywhere. Don't take my word for it. Try one and send it back if you don't like it. No tube anywhere let you do that.

Then before messing with perfectly good tubes on a perfectly good amp look at Cones, power cords, contact cleaners and conditioners. There's a whole long list of things to do all of which will be far more effective than tube rolling. Which is by the way what its called. 

Tubes are not even that big a deal even within the tube amp itself. Transformers are far more important in determining the sound. Caps and diodes, as well. Tubes though are easy to plug in and out. They can be changed. So audiophools change them. Because they can. Which is exactly as good a reason as it sounds- no reason at all.
Tubes though are easy to plug in and out. They can be changed. So audiophools change them. Because they can. Which is exactly as good a reason as it sounds- no reason at all.
The same apply to FUSE?