I once got my hands on some NOS Mullards at a garage sale for cheap. I was so excited, couldn't wait to get home and drop them into my amps and just bathe in the glory of vintage goodness that would now blast out of my amps.
Guess what... the difference was really, really subtle...
Well tube swaps ain't going to change an amp dramatically into a different one...so yeah, if it's a Blues Jr, it will still sound like a Blues Jr as you say....but different tubes can make a difference and can help fine-tune the overall sound quality to your liking. Also, some amps don't "change" as dramatically with tube swaps as others can. It's all about experimenting.
Just out of curiosity, was that the only time you ever tried something else in you amp beside the current "Groove Tubes" stuff, which is mostly Chinese or Russian current production tubes...?
Not everything that has a famous brand name is going to have an outstanding effect, but yeah, like you said, you did notice some improvement with smoothness.
Also, unless you can test the tubes before even putting them in your amp...you have no way of knowing how good the tube is.
I don't think people need to loose their minds over tubes, but sometimes the stock tubes do suck and can make an amp harsh/fizzy sounding, so you don't really need to buy a different amp, you just need different tubes.
For the large amount of tube gear that I have between a bunch of guitar amps, rack gear and mics....I acquired a decent amount of tubes over the years. Lots of NOS stuff back before people started going insane with their prices.
I also have a decent tube tester to check and grade the tubes.
When I decide to try out some different tubes in an amp, I'll spend maybe a couple of hours experimenting with various combinations in the pre, rect, tone and power sections of a given amp, and sometimes it's exactly like you say, very subtle from one tube to another, but then there are some combinations that are quite noticeable (either in a good or bad way).
It's actually fun to experiment, though you need a good amount of spare tubes on hand, otherwise you can't get a sense of what a tube swap can do to a given amp from just a couple of tubes.
Oh...and I actually do also use current production tubes in some of my gear, so I'm not snobby about "NOS" tubes....I just like tubes, so I keep a good stash for now and future use.