So...I build and restore old guitar amps as a hobby. Anyway, one of the first things I learned when I started building amps is that coupling caps need time to burn in. When I first fire up a new amp it always sounds a little soggy to me. But after some hours of playtime the amp both stiffens up and gets more focused and darker in tone.
Whenever I hear a story like the last post, I think they are hearing that too and instead of riding it out and letting the new caps burn in, they switch back to the old ones.
On a unit that old, I think it's crazy not to at least replace the lytics. Truly, with them, it really is simply a matter of time.
As far as coupling caps go, over the years I have learned that some types of caps just HAVE TO BE REPLACED while other types seem to hold up fairly well. With 50s amps you just have to replace the coupling caps. I have not ever one time found a Sprague bumblebee or an early Sprague Black Cat that was actually in spec...ever. Pyramids and Astrons are hit and miss. So much so, that I always replace them because I am lazy and I don't want to do repairs twice!
At some point in the early 60s Fender switched to different caps. The blue caps with the white labels that designate their ratings are usually ok. Since your Champ is a 66 I'd bet that's what's in it. Personally, I'd probably leave them in there. But you really should replace the paper bias cap and the three big lytics.
It'd be a real shame to damage an amp that's made it 52 years now for 20 bucks worth of caps.
My two cents.
Sorry if it sounded preachy!!