What cab to go with Eden wt300 and Bag End s15b-d?

Well, there is the typical solution.......

 

A BE 12 would go quite nicely with the S15 you presently have.
In fact I run that exact pair with an Ampeg V4BH and am often complimented on the tone heard in the room. They present a nice 4 ohm load to your WT300.

 

If you don't plan on using the S12-D separately as a standalone for smaller gigs, then it definitely sounds like a good solution. (And of course, a S15-D would indeed be the obvious solution.)

I'm in the same situation: I have a S15B-D and I'm looking for cab to pair it with, but in my case, the cab needs to be a good standalone. I've thought about adding a S18-D since I heard it's pretty punchy for an 18 and that it's sufficient as a standalone. I bet this would be a pretty rockin' combo.

 

I'd say Bag End S15X-D. I had one some years ago. It's got a tweeter in the center of the 15, but it isn't a harsh sounding tweeter at all. It just clears up the sound - it sounds quite natural.
Two Bag End 15" and an Eden would be killer!

 

A big +1 on that... the 15d and 15dx make a very nice pair. The only thing I'd worry about is that there is no tweeter attenuation contol. It is a very nice, natural sounding tweeter, but it does put out some high end sizzle. If the thread originator really likes the sound of the 15d... why not just another one?