Just a quick mention that if you have the Exicon 10P20 upgrade done to any of our hybrid preamps, this will be more of a major musical improvement than your choice of any of the tubes discussed herein.
Based on the observation that a clever thing to do is to locate and fix the worse case problem first, it certainly appears now that the Hitachi J79 was the worse case problem previously. It took lots of years to find something a lot better, and now, unfortunately, the 10P20 plastic case device is out of stock for probably four months. We have adequate inventory for new products, but not for any more preamp or DAC upgrades until they are available again. I wonder if they are a victim of the SE Asia horrible floods this winter?
Frank