Since you are diggin on the ET65, then yes, you should love it in a closed extension cab. It will be a little more solid on the bottom than in the open cab.
I have a chance to pick up a WGS G15 C for a very fair price. 15" speakers are rated good for jazz in general, but anyways here we go: Will this speaker be good for jazz and be a good companian for the ET65 ?
Sure, it would be great ... a more round tone than the ET65 ... plus a bit brighter, too. If you go G15, I'd go with an open or semi-open back cab ... it needs a lot of air to breathe properly.