Synergistic Red Fuse ...

Jon, sorry that you’ve experienced this issue. I have no particular suggestions beyond what Sgordon1 has said (although you indicated that you’ve already performed step 1 of his suggestions), but the following may have some relevance:

I took a look at the detailed technical info for the 0.25A 250V 5x20mm fast blow fuses manufactured by Littelfuse, one of the leading makers of non-audiophile fuses. Interestingly, they offer at least four different series of fuses of that specific type, designed to various combinations of UL, IEC, and other standards, which have "nominal melting points" (defining the combination of current and time that will cause the fuse to blow) ranging from 0.0166 amps squared-seconds to 0.042 amps squared-seconds. That’s a difference of a factor of 2.5, even though all of those fuses are from a single manufacturer and all are 5x20mm fast blow fuses rated at 0.25A 250V.

If the stock 0.25A 250V 5x20mm fast blow fuses in your amp are rated at or near the upper end of that 0.0166 to 0.042 range, and the unspecified rating of the SR fuse is at or near (or even below) the lower end of that range, it would certainly seem to account for what happened. If you call SR, you might ask them if they can tell you what the nominal melting point of that fuse is, in amps squared-seconds.

Good luck. Regards,
-- Al

One would hope Synergistic will do something to help out in a case like this.    It is an expensive experiment for someone and not user fault if apparent correct fuse blows unexpectedly.  One the right fuse is found, should not be a common occurrence anymore I would hope.

"It is far, far from my intention to cast a pall on SR product, as I've greatly enjoyed tweaks for my setup over the years and was very much looking forward to this one, and it's not in my nature to be a downer on something so fun for all of us."

Hmmmmm, interesting.

The main function of a fuse is to provide protection. I'm inclined to agree with Mapman and Ozzy in that the SR fuses could be  made to err on the side of being perhaps more sensitive and easily blown(Al's example of specification range). Better to sacrifice the fuse rather than damage the components. It seems that the SR protects with a quick trigger in addition to sounding far superior to the stock fuse. 
Charles,