Any tube can fail at any time, never forget that. Even new right out of the box. Just because one set of one brand lasts five years doesn't mean the next set of the same exact tube won't die in a week. Never forget that.
You note that the 6L6 and the 5881 have subtle differences in tone. Well so do each different brand and variant on 6L6. Hard to say that the differences you hear between one 5881 and another 6L6 are due to the number on the label rather than finding the same amount of difference by switching from SOvteks to JJs, both 6L6.
6L6 - of which any current production 5881 is one - has a different bias need than the EL34. Now each type has a range, so it is possible one EL34 needing more bias will work with a 6L6 that needs less, but as a rule, mixing them is not kosher.
Bias won;t adjust down far enough? Change a couple resistors to widen the adjustment range.
I have a question. YOu said you inserted a current meter in the cathode leg. was your 49ma per tube? as in 98ma reading?
49ma tube current at 453 volts is about 22 watts disipation. A bit hot for my taste.. I'd raise the bias voltage some to bring that down.
A word on 5881. If you want to consider specs, look up the data sheet on the actual 5881 you are buying (or considering at least). Don;t look up 5881 in the old RCA guide. Unless you are buying NOS tubes, the 5881 today is not the same tube as the one in the old tube manuals.