It's never easy to be sure what caused something like this. Nevertheless, I'll offer a piece of science ...
With typical tube power supplies, the initial rush of current to charge the capacitors is much higher than the steady-state current. That's the most likely time to blow a fuse, if the amp is functioning properly.
The total energy in that initial burst is proportional to the current and to the time it takes. The fuse also has a time scale, related to the thermal mass of the element and the melting point of the alloy used. Slo-blo fuses have a larger chunk of metal so it takes longer to blow.
Fuses use a variety of alloys, depending on the current rating. The unfortunate thing is, that different manufacturers use different alloys, or choose the break from one alloy to another at not quite the same current. So all 1.5-amp fuses will blow eventually at the rated current, but there will be a variation between manufacturers in how long it takes at higher currents (i.e. the inrush current). If the inrush current drops before the fuse blows, then you are good to go - but an identically-rated fuse from another manufacturer might not survive the inrush.
If you look up some data sheets, you are likely to find a rating for the impulse, usually given in amps squared over seconds. For a very egregious example, a Littlefuse 313 03.2 is rated 3.2 amps and will blow with an impulse of 209 amp^2-sec, whereas their 313 004, rated 4.0 amps, will blow with an impulse of 76.1 amp^2-sec - it will blow nearly three times faster, in spite of the 25% greater rated current.
