Those two sentences seem to indicate total ignorance, from the manufacturer of any amp. Not surprising, from those with the hubris of McIntrash. Of course; that's just my own personal opinion, based on personal experience with both.
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.