got a crate cr-60b half stack and have questions about cab extentions

[c-had]

I just got a 79 crate cr-60b with matching cabinet. And i love it, its warm and clean and has descent volume for a 60 watt amp. I also have a peavey tnt that is 200 watt when you add speaker extension. I use the crate cab with it tonight and it sounded good. Both are 4omhs. My only concern is the crate 15" cab says 75 watts max on the back. I wouldn't think any 15" driver would only handle 75 watts. Can anyone help? thanks Chad