Gee, for a 6-year-old, you type pretty good :D
Give yourself a few more years and you'll be used plenty!
Seriously, in a word, YES, you are probably due for a set of output tubes.
Find yourself a decent amp tech in your area and get them replaced and BIASED. This will ensure a long, healthy life for not only the tubes, but the output transformer, too. Your tech will probably recommend whatever he can sell you. If the amp has EL34s, I would go with the JJ (formerly Tesla) brand. Beautiful cobalt crystal glass, they hold their vacuum very well. Otherwise, Groove Tubes offer a range of "matched" tubes that will
yield "soft" to "hard" response, depending on the grade you select.

I'll 2nd the Tesla/JJ's recommendation. Amazing tubes and an incredible value. The Ei's out of Yugoslavia are very kewl too. They have excellent output and sound alot like Mullards and of course NOS Mullards are fantastic (and my fave's lately). JAN-Philips are very exciting NOS tubes as well. They have a very bright and vivid color to them that has amazing presence.

Yep, what they said....pre-amp and power amp tubes, a matched phase invertor, bias, ect ect by a good tech. And if someone tells you there's no real difference in tubes run out of the shop :) .

And don't forget to have the filter capacitors checked. And check for loose ground points. Those two items can rob punch and tone, and get overlooked by do-it-yourselfers and pro's alike. 8)

A little late on the response---but through the yrs---Ive done alot of trial and error with trying out tubes for my Marshall,and I must say that I always lean towards Groove Tubes.