The flabbing out on very low, very fast notes might not be the pickups or the amp, at all. It could be the speakers. I just changed my 5150 cab from sheffield 1200s to Celestion G12K-100s and EVM 12L black labels in an X pattern. At bedroom volumes, it's a LOT more articulate in the bass end. I'll crank it for the first time tonight, so I'll let people know what it's like at volume soon! They still need to break in, of course!
Roo