why do hi-end fuses keep blowing, while std don't?

I'm interested to hear what the fuse maker says.

Good idea.
Change $5000 amplifier for a $25 fuse.

McIntosh's advice to check tubes if fuses are blowing in general is hard to argue with.

No vendor will support a product that the user has altered. Right or wrong, if the vendor has doubt about use of any part introduced by a third party, including a fuse, for whatever reason, including lack of information, they are NOT going to recommend it and then be obligated to support that recommendation.

Also, the comment about whether the fuse can sound better or not might be a bit strong, but I'd have to take the position that the purpose of a fuse device is NOT to make something sound better, even if in fact it does. THere are many other ways for a product to accomplish that. If there is any question about the product being able to serve its primary function, which in this case is to PROTECT the gear, not make it sound better, then a no confidence confidence vote for that part in a case like this which seems to support that concern is not surprising.

That response increases my confidence in McIntosh support, it does not decrease it.

SO there is a choice to be made. Making sure the tubes are in good working order would seem to be a logical next step. One or more sub par tubes may well result in lesser performance and/or other issues, regardless of fuse used. Having tubes all in good working order is paramount to a tube device working as it should. Flavor of correct fuse used matters so much less, to the extent it even matters at all, that it is a no brainer which to address first.

Excellent response Mapman.

If both the standard and upgrade fuses were blowing, the
tubes would be the first thing I would suspect and test.
Since only the upgrades are going, and especially given that
the tubes are new(not that new tubes don't/can't cause
problems), I wouldn't waste my time there. Personally; I'm
using eight(what were NOS) GE 6550A output tubes, in my
Cary's. I've had two Hi-Fi Tuning Supreme fuses go out in
the(fixed)bias protection holder(one each amp over the
course of 30 days). I'm still using Supremes in the mains,
but- though I really like the more natural presentation of
the upgraded fuses; I can't afford slightly over $90 a
pop(literally), for the bias circuit. I tried a pair of
Synergistic Research Quantums in the amps and they held
fine(didn't care for the sound). Reinstalled a Littelfuse
pair of the same value, a couple months ago and no problems.
Has anyone mentioned that Hi-Fi Tuning Supremes are rated
250V, Synergistic's are rated 500V, and many output tubes
use 400 to 450V, plate voltage? That would only
matter in one's bias protection circuit, of course.