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

Showing 2 responses by three_easy_payments

Absolutely absurd dismissing that tubes can make a difference while gushing over a fuse. I’ve rolled tubes and I’ve tried Synergistic fuses and other products from SR. All have made a difference but I have to say, a pair of NOS 7308 Mullards from Brent Jessee moved the needle so much more in my preamp than dropping a SR fuse in the same piece of equipment. We shouldn’t be preaching "the only thing that matters is what you hear with your own ears" while dismissing what folks hear with their own ears! This type of cognitive dissonance is really getting old. We are all here to share opinions but self righteous behavior has no place here...it's unhealthy and unkind.
In the big scheme of things though you will hear much more difference changing the fuse inside the amp than changing the tubes.

This is a gush, this is dismissive....this isn’t "correctly pointing out" anything....this is opinion that is dismissive of anyone else’s experiences that they wish to share. 

By the way, I never refer to anyone who lends an opinion as a "hack".  Who does that? Seriously???