I don't think it is the soil- I've grown strawberries in different parts of the yard, including raised beds. I've also bought them at farmer's markets and notice the same thing, so it could just be that in this climate they don't get enough sun. Some of the past ones I've grown (different varieties) and the FM berries were sweeter than the Cabot, but still lacking. More tender than the store berries, but not more tasty. But, it could be the varieties that are grown here.
I haven't tried MDB, but I did have Caroline raspberries for a few years. I got rid of them, as they weren't all that productive and were heavily infested with SWD for a couple years in a row (Prelude and Royalty fruited before SWD and Anne seemed less susceptible at the same time). But, the taste was probably the best of all the red raspberries I've tried (I like Anne better).
I figured that might be a reason the CA ones have an edge. Cabot is a mid-late ripening variety. I think it actually has less sugar than the earlier ones I used to grow (they used to be around June 1st, while Cabot was ~June 15).
Do you have a good source for Mara des Bois? Given that both you and MrsG reccomend it, I think it sounds like a good one to try. I checked both Nourse and Indiana Berry and while both have Jewel, neither has MdB. One note- Nourse says that Cabot has "Excellent" flavor, which makes be really wonder about the ones they say are only "Good".