Is blending power tube brands recommended?

Hello tube owners who are wiser than me!

I have a vintage Fisher 500c and have been running a matched quad of current production EH 7591s in it with fine results. However, I still have the original quad of Fisher branded 7591s that I believe actually came with the unit circa 1965! They still test strong.
So, on a lark I threw the Fishers back in and was quite surprised at how good they sounded! Warmer, much more "PRaT", and just more "liquid" especially in the mids. Soundstage was a bit shallower, they're a bit less detailed, and not as quiet, but I really enjoyed the change. EH tubes are still better in some respects though.
So could I run a pair of each safely and would I get the best of both sonically?
I'm no expert, but yes, seeing that they are all 7951 it is safe to try. 1 EH & 1 Fisher on each channel set up to mirror the other channel.
Yes you can do this but you could wind up with the weaknesses of each rather than the strengths. Just try it and see how you like it
I would think that power tubes would need to be biased wheb they are put in. If you change them, you would need to set bias voltage for the different tubes. I trust you noted the locations of the oeriginal tubes. If your unit is not biased you will need to do so, as you could burn up your power tubes more quickly.
I have some Fisher tubes made by Telefunken (look for the diamond on the base), they are very good. I would not mix brands in a push pull situation though. Swap them out as sets and make sure you check the bias.
I had a 500b model and used the original Fisher branded 7591....if there is a " DIAMOND " logo on d tubes..then it's a diamond sound !! " Telefunken "!!

Btw keep your EH tubes for back up.

Enjoy the music.
Thanks for the input, everyone.
The 500c is autobiasing, and honestly, I wouldn't know how to check the bias on it if I wanted to. I should learn more about that!

After doing a bit more research on 7591 tubes, it appears that just about all of the original iterations were originally manufactured by Westinghouse, and rebranded. So I'm very curious about the possibility of Telefunkens in the 7591 tube family?

Heyraz and Pescolar, are you sure you're not confusing the 7591 and the 12AX7s that are also common in old Fisher gear? Most of my 12AX7s ARE Tele smooth plates, but not the 7591s. Not trying to question your wisdom, just hadn't heard that Teles were even available in that particular tube.

If it's true, do you have a source for them. I do love the Tele sound with the different tubes I've experimented with.

At this point, I think I'll keep running the vintage 7591s I've got, and keep the EHs for back up. Definitely prefer the warmer more dynamic sound of the oldies, even if they are just a tad bit rolled off.

Thanks! And any other thoughts are welcome!
Lincoln- Sorry about that.. The Fisher Receiver I (briefly) had was missing it's power tubes (Estate Sale). Apparently I was in error when I assumed they were Telefunkens just like the 12ax7's. My bad. Good luck.