Yep, there are difference in tone and character of each tube you listed. Note that the STR442 are NOS SED =C= EL34 tubes. No worries as they are sold out so you can't order them by mistake. Did not know the Filmore could use EL34 tubes.
I think you meant the STR441.
The run down of Russian tubes: STR441, STR443, STR425
The STR441 is a mild tube, not bad in character. Sounded good in the MWDR. Did not like them in the JP2C as I felt something was lacking. This tube is similar to the Mullard or Tung Sol 6L6GC-STR.
STR443 alternative to the STR440 Chinese Shuguang tube. Not bad in the JP2C but low end is not as aggressive as the STR440. This tube is similar to the Tung Sol 7581A. Hard to tell which it is. The base looks like the Sovtek 6L6WXT, Electro Harmonix 6L6GC. Very similar plate design.
STR425, Basic 5881 tube. I have tried a pair in the TC-50. Had a better response than the STR440 tube.
Tubes made by JJ Electronic: STR445 Have not tried these tubes.
STR448 are Chinese tubes made by PSVane for Tube Amp Doctor (TAD). The red base is also making the claim on similar to Mesa STR415 tubes. Before this, it was the Shuguang tube with black plates. An RCA replica of sorts. I have used those in my Mark V90. Nice tubes. They were replaced with the Red Base as Shuguang is not manufacturing until after relocating. Have these on pre-order. Waiting.
