Blown tube

I installed a new set of matched KT88s not too long ago on my Rogues (couple of months and maybe 350 hours or so ago)...
I've had a tube blow and one other on same mono amp stating to blacken on its top..
What might cause this so pre maturely?
Perhaps your BIAS?
I just had two go out in a lot less time.
...self biasing, mmm.
Blown fuse or?
I did notice when amps arrived, the same socket/tube was not working, so....maybe socket?
Power tubes do short out on occasions - some brands are more prone to this than others. You are fortunate if all you got was a blown some amps you will take out a bias resistor and/or a fuse. The only way to avoid tubes shorting out is buying tubes generally noted for being rugged in the first place.

As suggested above you could be running your tubes too hot. Are you using the manufacturers recommendations?
Just noted that you said your amp is self biasing --FWIW I've a self biasing amp - put in a new set of tubes and one tube's plates started to glow red. Changed it out thinking it might just be the tube, the next tube also glowed red. Returned the amp to the manufacturer and a minor adjustment to the bias circuit solved the problem.
Thats a common thing in rogue amps. they are hard on tubes, factory Highly reccomendeds FACTORY matched sets only on outputs. those amps are great much better then most people might think. Ive spebt 2-3 times as much on 5 sets of mono blocks only to sell and buy new 120's.
hurricanes, vtl mb300, rogue zeus,ect
Tubes tend to die in two ways: (i) infant mortality, in which instance they die in the first 200-300 hours, or (ii) they live until the end of their normal lifespan. They rarely die in between.

You are experiencing infant mortality.
Amazing, thanks for the input. This is all valuable info for me.
bad amp