I have the 9.0x. I’ve tried every current production and NOS tube under the sun, except Telefunken. Here are my favorites in order of preference ( some with Herbies Rx tube rings and some not. You must experiment for your rig. System audio is professionally calibrated. All tubes listed excel in tonality!
Valvo 6922 1960-64 ( same as Philips miniwatt Holland) and PCC88 (7DJ8) A frame or non A frame . ( These tubes fire on all cylinders across every genre of music I throw at them. Superb. You can save money by getting the PCC88 versions. They are safe to use per Modwright and will likely last a long time due to their 7v heaters only receiving 6.3v from the 9.0x. Very very close sound to the E88 cc versions.
Matsushita/Mitsubishi 7DJ8 tubes. I believe these were made in the 70s or 80s.
(very musical. If Valvos are a 5th row seat, these are a 10th row seat. I like both approaches)
Siemens E88cc (a nice all arounder tube)
Bel India 6922 1960s (made with Philips tooling. Very musical, engaging and balanced)
Amperex 7308 JAN USA 1960s ( deepest tightest bass, but others listed above are also excellent for bass)
Amperex 7308 Holland ( small-ish soundstage compared to those above)
Mullard E88cc ( too warm. No sparkle, but a good choice for a bright system)