I would also consider breaking out the pH meter to see if this is a "true tartness" (i.e. the beer has low pH around 4) or if it's a percieved tartness. If you really have low pH then there might be ways to adjust your water chemistry. See the pH effect on finished beer thread.
As for the yeast, I brewed 10 gallons of hefe recently 1.052 OG, 60% wheat, 35% pils, 5 % caramunich II step mash 35 min at 145 45 min at 160 then mash out, 15 IBU's from 1 60 minute hallertau addition. fermented in two seperate carboys - one with WLP 300 the other with WLP 380 at 62degrees then blended to balance banana and clove.
Scored 40 points in a local comp and tartness is mild.
Pure genius! I swear I spent half of this past summer trying to perfect my hefe by trying different yeasts, pitching rates, etc. They were all damn good beers, but not quite what I was looking for on the clove/banana balance. I'm definitely going to try blending two batches on the next go around. Thanks for the inspiration!