I decided to dink around with tubes in my Hi-Mu head that has the air-gap OT (7 watts output through a 2x12 with two Weber ceramic Cali 12's) I started with one of my favorites, a Hytron 6V6GTY and a Telefunken 12AT7 and a Telefunken ribbed 12AX7. It's been that way for quite awhile and sounds superb. Then I pulled the Tele's and put in a Mullard 6201 (NIB) in place of the 12AT7. That stayed throughout. I put an original 70's Tesla ECC803S liked it a lot more than the Tele for the 12AX7. Then I replaced the Hytron 6V6GTY with a Bendix 5192 (very rare tube). That tube sounded fantastic with the Tesla. I then stuck in an Emerson branded 6V6GT (not sure who made it, USA though) and it was the best so far. Thick, not too dark, articulate when driven and sustain to the max. I then popped in a shield Mullard 12AX7 and that really made the difference with that Emerson. So far this is it. The Mullard 12AX7 and 6201 stay from then on. I only mess with 6V6's now. I then stuck an RCA black plate in and wow, they sure are up to their reputation. So far now, this is the best combo. Then I tried a Mullard 6V6GT and believe it or not, this tube beat out the RCA by a small margin. More presence and "bloom" whatever that is. I went back and forth for awhile with the RCA BP, Emerson branded and Mullard. It was tough but the Mullard was still my favorite. Then I decided to take one of the two pairs of Brimar CV511 6V6GTY's and split the pair to try one. HOLY FRIGGIN MOLY! BY FAR, this tube with the two Mullard preamp tubes is the holy grail. The presence and thickness and articulation is on par with the RCA BP, Mullard and Emerson branded but just a bit more there but then the SUSTAIN! This thing sustains like it has a pedal with the Brimars in it. Holy Cow! What a difference. Just thought I'd share.