Would also love to hear about differences people have heard when using higher end amps / preamps with the rarer, and less obtainable NOS tubes.

I’m currently running a set of first issue Telefunken 7DJ8 and a set of Philips 1955 EL34 metal base in an amp. I am thinking about trying to find a set of unobtainium Siemens CCA or the like, but just not sure if they’re worthwhile if I could find them.