Not tried the 5881 in a Leben. However the plate voltage in EL34 and 6L6 modes are 414Vdc and 450Vdc respectively. This is more than the spec sheet max of 400Vdc for original Tung Sol tubes. Other manufacturer’s 5881 which claim higher plate voltage may not be 5881 but rebranded 6L6GB etc. I have tried NOS TS 5881 in my 6L6GC vintage monos and I couldn't bias them down from slight red plating so I didn't push it and took them out.
