Gents, I am finally able to pay more attention to this thread.
First, jjazzyj, I want to apologize for the previous post. Whoever put on the EHHA website that 2.5" heatsinks were adequate for speaker duty is an idiot. The best explanation I can offer is that I wasn't really paying attention.
You guys have mostly sussed this out, however. At 18W into 4R each mosfet will burn about 18W itself. This is they typical effiency for class AB (mostly B) operating condition.
If we can live with a 40C temp rise over a 25C ambient this will put us at 65C, ten degrees less than the halfway point to 150C.
Now I know we won't have ambient but just to get a ballpark number this is 40C / 18W or about 2C/W. Given all the various thermal barriers between the junction and the air I think that Marc's recommendation that the heatsink itself needs to be 0.5C/W is a good one.
And, of course, only off-board heatsinks can do this.