How much of a difference in rating is ok for speakers in a cab?

I believe as long as each speaker is rated more than 20 watts handling you will be fine.

 

Yes, steve, oversimplified that is how it works. People smarter than me can tell you about other factors like ohms and amps and speaker efficiency which factor in as well.

 

In multiple speakers set ups you multiply the lowest wattage speaker times the number of speakers to get your power handling capability. For instance in a 2x12 cab if one speaker is rated at 100 watts and the other is rated at 30 watts then the cab will be rated at 60 watts. Why? because it will recieve have the power of an amp its attached to and when it gets to 30 watts its reached its rated limit. The other speaker wont "take up the slack" so to speak. In a 40 watt amp then the lowest you would "safely" want to go is twenty watts on one speaker (or both). At the moment I have a 150 watt Tonker and a 30 watt G12H-30 hooked up to my amp. Hoever I know I cant push more than 60 watts thru them safely. There is no problem having a big difference between them in power handling and they are matched pretty well in sensitivity. Bob

 

Yes.
The lower wattage handling of those speakers is the PJ, which can handle 50W. In a 2x12 it would only be getting half the power from your 40W amp (20W). So that is a very safe pair of speakers to choose for a 40W amp.

Oh, please make sure you match up the total ohms of the cabinet to the output of your amp. Please ask if you are not sure about this.