Roadster tubes, 6l6 vs 5881.

Yes I know I've been a little neglected but well...

I bought a new Roadster head that came in perfect condition 2 years ago. At the time when I was checking and testing the amp I notice that one 6l6 tube made an unusual sound when I tap it, like something was kinda loose inside. I put it back in the amp and keep using it for recording and rehearsal purposes since then with an eye open in case something change in a bad way. Well, nothing had happened, the amp sounds better the ever.

Now, with that said...

I'm looking to buy new tubes for the Roadster as a precaution (no Mesa's EL34, bought them before and for me they add to much fizz and makes channel 1 & 2 kinda sterile), made a research and ended up with 2 options (don't want to use none Mesa tubes for now) the stock 6l6 STR 440, witch I like a lot, or the 5881 SRT 425.

1- Have you checked them before, how they compare with the stock 6l6 440?
2- Can I use them with the Roadster as a direct replacement for the 6l6 440 with no harm whatsoever?

Thanks in advanced guys 8)

If you use Mesa tubes, they will be a direct replacement.