Joe
Have you still got those NOS 6H23p tubes we bought from the guy on Ebay? If so I would try those first. They sound pretty good and will last longer in the GAT like they did in the AI preamps. If you want to try Mullard I would call Andy at VTS and prepare for "sticker shock." An alternative is the Matsushita tubes made in the factory Mullard personnel set up. They are pretty close in signature and half the price.
In what ways are they “better”.The 6h23p is a direct six volt replacement for the 6DJ8 family just a more rugged Russian military tube and was all they could find to work for any length of time in the Modulus 3 preamp. They are definately better than the new stock tubes CJ is providing.
I had the 6H23 in my original GAT, they sounded great and lasted a very long time.
If you prefer a little more neutral, cleaner and clearer sound, get the upgrade. Otherwise if you love what you are hearing and prefer a slightly sweeter more romantic sound, leave the GAT alone. I know of owners going back to older or original CJ pieces after selling or upgrading to GAT2 or new ART amps. I also know many who absolutely love the GAT2 and ART150/300 amps. It’s all personal taste.
For the record, I’ve never had any tube issues with either my GAT or current GAT2.