wrote:
Is it possible to replace certain octal tubes with equivalent miniature
in an older receiver

For the most part, but it depends on the "certain" tubes.

and would there be an performance improvevment?

No, not really. If anything it would probably be a lot of work to get
things so the performance is not degraded.
--scott

--
"C'est un Nagra. C'est suisse, et tres, tres precis."