A tube seller who I shop agrees that most current production EL34s do not last as long as those of yesteryear – with one exception: the EH 6CA7, which he feels can definitely take the stress. Last time I checked, on his website I believe he points out its sheer increase in plate area and extra cooling fins vs. EL34s.
As far as current production EL34s, I am kind of inclined to say "just let 'em burn." If you don’t use your amp every day, it might not end up being much of a problem.
But, as it has already been said in threads like this one, we might not be able to hear much if any difference by lowing the bias a bit (though I still feel kind of reluctant to do so).
If you have an original Stereo 70 having a PT wound for 117VAC mains, then one or two (maybe even three) appropriately rated thermistors in series with the primary should help to drop the internal voltages to whatever level you feel is appropriate. If it’s too much of a reach for thermistors because your line voltage is too high, then some folks run off of a variac or bucking transformer or whatever. So, that part should be solvable one way or another.
The Dynaktparts.com PT, last I knew, is wound for 122VAC mains, and I have one in my dynakitparts Stereo 70. With this newer transformer, and a thermistor, my 122VAC-125VAC mains results in something less than 400VDC (original spec=410VDC on plates with 117VAC mains) on the plates of my EL34s, so I can run them at 50mA with a reasonable level of power dissipation in the plates that I would think should result in a reasonably long lifespan for the EL34s.
If other amps like Jolida and others nowadays are running their bias lower, I suspect it is also to reduce the likelihood of warranty claims. But, we’d also need to know how their plate voltages run to get an idea of the kind of stress their tubes are actually getting. For instance, my Cary SLI-80 sets the 6550/KT88 to about 35mA apiece, but, from what they have told me, the plates operate somewhere in the vicinity of 550 volts. Even at that voltage, it seems like there should be room to turn the bias up some...