Although my old Cronus Magnum was electrically compatible with the KT120 [per Rogue] I needed to change the chassis top. The older chassis top hole cut outs were too small in diameter for the larger base on the KT120. I purchased a new chassis top from Rogue years ago.
I didn't have much choice but to run the more powerful KT120 with my Thiels speakers. Now that I'm back to my more efficient Tannoys I am able to experiment with a wider variety of tubes.
I have a quad of EH 6CA7 that sound wonderful, overall much nicer than the KT120 IMHO. On the other hand, I just got my fresh quad of JJ 6CA7. These JJ's are very robust and deep in the bass and lower freqs in general but they don't have much top end. In a word they are rich and dark. Ok two words. I'm leaning more toward my EH 6CA7 which are more balanced sounding.
