The Viva Fono at $6k is a pretty good bargain, considering that it lists for $16k euros. I own that phono stage and I like it very much. I have replaced the stock 12AX7 with Telefunken ECC803S tubes. In the past, I have used it with the .30mv Transfiguration Orpheus L feeding into a linestage with 18 db of gain with no problem. With my current linestage which has less gain (gain unknown) I have no problem either.
You probably have enough gain for a .24mv cartridge, provided you crank the volume a bit. This is a dead quiet phonostage so you can push the volume up without a noise issue. It is also a really terrific sounding phonostage.
I have heard an Aesthetix IO that was a bit noisy sounding, but, not to the point that I thought that noise was a big issue. It might well be the case that different/replacement tubes would lower the noise level. I only note that trying to provide a whole lot of gains going with active, tube amplification is more challenging, when it comes to noise, than providing some of the voltage gain via a step up transformer, which is what the Viva fono does.
One more thing to note. It is NOT easy to change the loading on a Viva Fono. That is done by soldering in a different set of resistors found at the input jack (this means primary side loading). The good news is that this phonostage is NOT particularly sensitive to loading changes with either the Lyra Titan or Orpheus L cartridge that I use. I hope that would be the case with the ZYX as well.
