replacing a transformer with one of slightly more voltage
If you actually need 5A, then the 90V one is under-powered and could overheat.

The difference between 96V and 100V is only 4%.  This is less than the tolerance in a domestic mains supply (possibly ±6% or ±10%, depending on where you live).

You're unlikely to notice any increase in volume.  If your mains supply is close to the top of its tolerance, then in the long term you could be stressing the components of the amplifier.