Rodman, he doesn't want that answer.Ooooo, baby! Why don't YOU tell me what I WANT to hear . . . that's soooo hot . . .
Anyway, I'm taking you up on this, Rodman . . . I ordered two Hi-Fi Tuning 2A slow-blow fuses from Acoustic Sounds, they have them at $34.95 (still made me wince a bit). I will do as you suggest, and use them in the AC line of my Marantz Model 2s. I'm thankful that your suggestion is for the easiest fuses to get at - gotta love external fuseholders right on the front ;)
I'm curious, did you change all of your power-amp fuses at once, or did you try it in one place, first? From what you described, the driver-rail fuses in the Hafler are the main suspect in an engineering sense, especially if these rails are taken directly from the main supply rails, and shared between both channels. I have personally seen (in solid-state amps) driver-rail bypass capacitors that "seem to be" fine and measure OK, but inexplicably cause an increase in measured THD. Same goes for cold solder joints on connectors that feed these rails.
If you were interested, you could change only these fuses back to the stock types, and see if that accounts for the bulk of the change in performance . . . but if you'd rather leave well enough alone, I understand.