Tube Suggestions Onix SP3

As a rookie to tubes, one of the Melody 5881s tubes on the Onix SP3 I recently purchased on Agon has gone pzzzsst and I'm clueless and suffering without good sound. I have to keep within a reasonable budget but at the same time have a solid replacement for resale value. Can the original Melody 5881's be had anywhere? Otherwise the tube shop I called said I need to replace all four, rather than only 1 and recommended the NOS Sylvannia 5881 at nearly $90 per. I'm tempted to just buy another used Onix if that's the case. (Just kidding) Help
Outstanding Tooter, Rhing and Newbee. Left a message with av123 checking on the replacement Melody as a plan B. I will work on the Bias per your instructions Tooter. Much Appreciation All.
Now there's more than one explanation for the distortion! On the right rear test the reading is 10.55 and won't go any lower. That's also the one that blew. I don't know much about this but something tells me I bought an amp from an Agoner with a bad 'wheel'. Can this be repaired or replaced in anyway? I'm O for 2 on Agon buys now - but with the great help from you guys I still have faith.

Any suggestions or should I post this as a new thread?
Are you sure that the reading isn't 1.055? Is your voltmeter set to the right "range"? Try swapping positions of the 5881 tubes and see if the same position gives the abnormal reading. You may want to just get in touch with av123. They've been very good aboout responding to emails/questions. I bought my amp second hand here on audiogon and when I had questions they were great to deal with. Best case scenario is that you just have a bad tube. Good luck, hope you get to the bottom of this!
I ran the test again before and after switching tubes and the reading actually went up to 11.23.

The rest are holding fine. Any idea what part is the contibuting factor? I will try av123 on Monday. I haven't had success getting an answer via email from them yet on finding the original tubes from Melody.

Thank you
The tube which went out, if it shorted out, could well have damaged some of the electronics in the bias circuit. This is not a rare event. I've had it happen 4 times on three different amps in the past 10 years. Probably time to take it into a repair shop (get some references 1st - not all repair shops know how to repair tube amps.)