Hrdx
There is a history with the Hrdx of cold solder joints on the pcb. These presumably started happening when the mfg. process switched to the new lead free solder and the overseas technicians did not have the higher level of soldering skill to master working with the lead free solder; which is harder to work with, as a lot of us have found out, requiring higher temperature to melt the solder.
They weren't getting the solder and joint hot enough and cold solder joint were produced which did not adhere well to the surfaces being soldered and eventually they crack and the current gets messed up at the cold solder joint - causing excessive heat and burning out of part of the pcb and some of the components close to the burn point.
I had my new Hrdx taken in to the auth. Fender repair place and Fender paid to have the cold solder joints resoldered and the amp re tubed and biased. All covered under the warranty. This procedure is highly advisable if you have not had it done - having a qualified tech check and resolder any cold solder joints on the Hrdx.
To make this inherent situation worse, a lot of guys have their Hrdx's further hot rodded by having the bias "set hotter" to get a hotter sound. Some claiming that the factory bias is set to cool for optimum performance; so they have the bias set higher, causing more heat at the solder joints and intensifying an already shaky situation.
With this fact in mind, I would carefully examine the pcb and look for yellowed out areas or obviously burned areas on the pcb board itself. There are pictures of the burned spots on that kid's Hot Rod Deluxe website that is still up but not updated, last I knew. He explains the whole situation there.
I would imagine the newer Hrdx's have addressed the cold solder joint problem and trained their workers in the skills necessary to make a good solder joint using the new solder, but turnover of workers is probably a big factor and learning to solder properly is not simple with the new solder and many of the new workers might not be capable of learning how to solder correctly.
Therefore, I would suggest checking all of the solder joints and looking for cold solder joints or having a qualified tech check and repair them, BEFORE you get the short that blows the power tube/tubes.
This is just one possibility. As noted it could be the other way around where a tube blew catastrophically and took out other components. But it can just as easily be the other way around where the cold solder joint cracked and overheated and burned out the power tube.
According to a Fender tech I talked to the Hrdx components should be readily available to repair any failures; unlike many short run, lesser known amps for which the Asian components are no longer available, rendering them non fixable.
Good luck with the repair and I hope it's just a case of needing new power tubes and a standard biasing.