No problem using six amp quick blow speaker fuses with the Ultra 550 amplifier. Although if you are blowing 5 amp fuses, you are putting over 200 watts into the speakers, and thinking about how much heat two 100 watt light bulbs generate, and putting that much heat into a voice coil, I would suggest that overdoing it could melt the voice coil wires. Caution!
Frank Van Alstine