Try E80CF in place of the 6U8. Only Valvo, Mullard and TFKN made them. I found some in eBay auctions that I paid $9 for. When I listened to the E80CF, I sold all of the ECF80s, and 6U8 types I had.
I bought a quad of Siemens E84L. After listening to them for a few days, I began selling all...