The 6L6GC being 30watt.... really a bit of misnomer, because it is - only in UL output amps, of which most guitar amps are not. The last big monster silverface Fender amps were, and are noted for their brittle tone, and terrible breakup. They do excel at "clean" though. So if you see a 60w Super Reverb (with no rectifier tube) or the same thing as a Bandmaster Reverb (MUCH rarer), or 135w Twin Reverb, or Dual Showman or Super Six Reverb or Quad Reverb.... dont expect it to break up at all in any sort of nice toned way.
The analogy for the breakup in these amps is like wanting some 4-wheelin fun on an old washboard road, and finding that you just fell off the edge of the Grand Canyon.
And they cant be changed back very easily or cost effectively.
There IS a good, no.. EXCELLENT tube out there for high wattage on the 6L6 pinout. Its the 7581. Those are like super-6L6's, that use the same everything - same bias, same heater current, etc etc, but with larger beefier plates and cathode. They were able to do this by holding very tight cathode-to-heater distance spec, and just generally upping the quality and keeping things really close together in the tube. The tube is built like a set of bolt on boobs on a fashion model. Solid and there! Expect to pay out the wazoo for them, but they're the top dog if you want clean, but good tone, in your 6L6 powered amp. That is, if you want to play bass through your Dual Showman (a wonderful amp for bass BTW), pop in 7581's and those low notes wont be a problem, because the amp has the iron for it.
Etc etc , your mileage may vary, consumatum est