Why are my mosfet fuses blowing?

I like Herman's call. Try other speakers.

I'll bet it still gets warm.
Re bias makes sense as do output devices breaking down under heat / stress.

The temp of the output devices is much hotter than the heatsink and all it would take would be some bad / devitrified heatsink compound to really cook some transistors.

Also, please check out this link to the CS2.3 Thiel. This is the measurement panel from the Stereophile test.
IF this is your speaker, I wouldn't use a bridged amp for any reason. It is a demanding load with a pretty beefy low impedance dip and a pretty nasty hi phase angle, to boot.

http://www.stereophile.com/floorloudspeakers/220/index7.html

I just went back to the Original Post.

I'd redo / re terminate or otherwise look at the speaker connections / wire.

This problem came on 'over the last year' which tells me something is changing.

Speakers also change from cool/normal to hot. The impedance and other characteristics change. These are consumer speakers, after all, not Pro Audio stuff which will take it for hours on end.

And yes, an overheating transistor can work fine while cool than go non-linear. If you can get at your output devices while the unit is warm / hot but before you zap it again, try a can of freeze spray on the heatsink.......cool off the heatsink which will cool the output devices. You may hear something from a bad transistor. I wouldn't spray the devices directly.