I use GE triple mica black plate 5751's and made a big difference in my Tempest II. The Sovtek 12AX7's sounded fuzzy and the GE's sound much more focused and clear.
Rogue amps-- try 5751 instead of 12ax7
I have an Atlas and just swapped out the pair of 12ax7s for 5751s and it's amazing to me the transformation. The Rogue (which I run with KT90s, biased up a bit) has been incredibly detailed for me, good bass, very transparent and I've loved it. Adding the 5751 (Raytheon windmill getters) has injected this really cool vintage-y tone and an incredible soundstage depth.... definitely at the expense of some punchyness in the bass but it's a very worthwhile trade off to me. I recommend giving it a try.