| I've never heard that. One of my basses has a single volume pot and a blend (balance) pot, but I never mix the pickups. It's got a MM in the typical sweet spot (near the bridge) and a single-coil J in the neck position. I play one or the other, never a blend.
It makes sense theoretically that a balance pot (especially a cheap one) could cut some of the harmonics more than running them through individual volume pots. But I've never A/B'ed a 2-pickup bass to try it out.
I suppose the trick would be to use a blend control that is actually two concetric volume pots wound in opposite directions, rather than a single differential pot. | 
