Tube Bias and other tube questions

I'm a tube type newbie looking for some sound advice. I have Quicksilver GE8417 monoblocks. I've had a problem keeping the bias steady on them. I had put new tubes in the amps when I got them and biased them (no input). I monitored the bias on them for several days until it seemed they had settled in. Approx two months later, one mono took a fit of snapping and sparking in an output tube. I changed the tube, but noticed the bias was off the scale (high). Setting the bias corrected the problem. I then checked the left amp. It was down from 110ma to 26ma! Questions: Is the tube that sparked likely to be bad or was it just overbiased? Why would the bias change so much after seeming to have settled in? How often should I check the bias? Would it be worth the Quicksilver price of $250 plus tubes to convert these units to EL34 tubes? Thanks to all who reply.
Unfortunately you have to monitor the bias frequently unless a self biasing setup.About the new tube,it will more then likely throw everything off.Check the bias once a week until it seems to settle.You should then check it out until it remains stable for a month.after that just check once every 3 to 4 weeks.Bias should only be off by a small margin.If not contact the manufacturer they should help you out.
The 8417 is a not a good choice. Good new old stock versions of this tube are expensive and hard to come by. The EL34 is a wonderful audio tube. There are lots of currently being manufactured versions of it at good prices. If you plan on keeping the amps I would do the change to EL34. My guess is this mod changes some resistors to allow for proper biasing on the output tubes. Find someone locally who can read a schematic and ask Quicksilver (Mike Sanders ) if he would send you the EL34 mod on paper. You'll learn about tube amps in the process and get a better feel for your amps and how they work. Oh, the 8417. I never liked this tube for audio. I think it tends to be unstable ( bias drift, etc.)