In my experience, there is substantial improvement over a 6DJ8 tube by going with a 6922 or 7308 which offer very tight triode sections.
Two top choices are 1960's Siemens 6922 and 1960's US made Amperex 7308 (either orange or white label). The Siemens is very clean, focused and slightly forward and dry, while the Amperex (especially the earlier white label) is holographic, airy, and presents a wonderful midrange. You could hardly go wrong with any of the vintage European tubes...
Two top choices are 1960's Siemens 6922 and 1960's US made Amperex 7308 (either orange or white label). The Siemens is very clean, focused and slightly forward and dry, while the Amperex (especially the earlier white label) is holographic, airy, and presents a wonderful midrange. You could hardly go wrong with any of the vintage European tubes...