Catastrophic valve failure.....

Gents,i could not turn away from this mint Steinhart tvp502 amp today(made me think about my good friend Leonard)....one channel down so i popped the lid and holy moly.......one pl519 has melted inward towards the plates and a pin prick hole has of course destroyed the vacuum(white snow all over)......i took the amp anyway because it really is in beautiful condition otherwise........what has caused the failure i wonder?.....can anyone help with a new quadset of 519's?....can the smaller 509 go straight in its place?
Thanks Andre.