CARY SLP05 - tube configuration?

The best tube compliment/combination will vary based on the mode of operation.

If single ended from input to single ended output, you only need to concern yourself with tube positions #1 and #2.

If you are running single ended inputs to  balanced output, you need to concern yourself with tube position #1, 2, 4 and 5

If you are running balanced inputs to single ended output, you need to concern yourself with tube positions #1,2,3,6

Balanced in to Balanced output you need to concern yourself with tube positions #1,2,3,4,5,and 6

Tube positions 7 and 8 are for the headphones.

Put the inexpensive Electro-Harmonix tubes in the tube position that is NOT in your circuit based on the above configuration

Mullard GZ34 are consider one of the best rectifiers made and are typically considered as a substantial upgrade over the stock JJ 5AR4. I use a 1960 Mullard GZ34 "fat base" rectifier in my slp05.

NOS Mullards run between $100 - $200 depending on year, manufacturing plant, and tube source (vendor). I always suggest going with a well known trusted vendor, you pay more, but you get quality. Also, rectifiers last a long time, therefore, the initial cost amortizes nicely over the tube’s lifetime.