12ax7 tubes can be enterchanged with xxx tubes?

I have not done substitutions in a Jolida, so my comments are just a general impression of various 12AX7 variants. For a warmer sound and big soundstage the Mullards and the Amperex are certainly good candidates. I have found that in a some applications, the Mullards sounded a bit phasey and the center image was not as well defined. In almost all applications, Amperex Bugle Boys sound great to me, though they can be somewhat pricey.

The blackplate 5751s are also a nice, reasonably smooth, yet dynamic sounding tube. They deliver considerably less gain than a 121AX7, so you need to experiment to see if they will work (in most applications that would not be an issue, it may be more of an issue in a phonostage).

I like the Telefunken ECC83, but, that tube is more on the lean, clear and dynamic side than on the warmer side. I run ECC803S in my phonostage, but, these days, they probably cost more than the Jolida (and they are VERY lean sounding).

The 1960's RCA 12AX7 that came with my phonostage sound quite dynamic and punchy, but, they are leaner sounding than the Mullards or Amperex tubes.

If one of the above posters is correct about the Jolida not pushing the small signal tubes very hard, then a purchase of new or relatively new premium tubes would be a decent investment because these older tube have PLENTY of built in life.

Good luck on your search.

Anybody tried mullard CV4004? Your opinion?

I have been using the 12BZ7s (gray plates sound smoother than black plates) in my Jolida for more than 2 years. Significant improvement in my opinion; smoother analog-type sound with more presence with my MG 3.6 Maggies. The 12BZ7s draw twice the filament current (300ma per tube) as the 12AX7 so I had to mod the 12 volt power supply to protect the 12V regulator chip that is rated at only 1 amp. The 12BZ7s worked fine without the mod but pushed the regulator to its limit. (No issues with plate voltage or current) The 12BZ7 is taller so you can't use the tube shields. The tip of the tubes just clear the top cover and protrude slightly through the vent holes.