Re: jj valves, ECC83S/ ECC803S
There are not. simple, black and white, answers to these questions you are asking.
Much of it is just conjecture.....opinion.
Do yourself a favor and delete the word "upgrade" from your vocabulary.
The best thing for YOU to do, would be to buy some preamp tubes, and see what you think. Replace them one at a time, and see what (if any) difference you notice.
Buy a 12AT7 and use that in the phase inverter (V3) and see if that helps your cause at all.......you can always switch back to an AX7 later.
TAD sells pretty good tubes...you just have to buy and try a few brands.
Biasing power tubes is going to be beyond your ability at this point...so just be aware...different power tubes of the same brand will draw Different Amounts of current... and may sound-play-feel quite a bit different within the same brand. Some companies will have a grading system...some will not.
Getting into THAT might be overwhelming at this point.
Bottom line.....buy some different brands and give it a shot.
Do not assume that the tubes in the amp now are any worse than another you might buy.
good luck