My first thought while reading this was if you just ran a speaker for a short period of time like 15 minutes what temperature does the pole piece go to?

That is important because if the pole piece would normally go to 138 in a short time this would mean there had been no build up of heat after an hour and a half. That would suggest you could just keep pumping at that level with no additional heat buildup.

The addition of the heat exchangers, crossovers with low end cutoffs and overload protection would make for a long lived speaker that would survive during concerts and from what I've read many don't make it now. Making "kits" to retrofit the most commonly used commercial speakers could be a new sideline in your spare time.

Grant.