I'd suggest that likely the higher distortion is due to the quasi-complimentary output stage (all NPN devices) design of the 9090, versus the full complimentary design (NPN and PNP outputs) of the 9090db. The positive NPN's on the 9090 provide a nice low impedence signal path to the load, but the negative side of course has to drive the load from the collector, which is a high impedence source. I've never been crazy about this scheme, but sometimes the result is pretty damn good.
FWIW, I measured the distortion on the last 9090 that I rebuilt at about 0.09%...quite a bit lower than spec. I would say that the last 9090db I worked on, the distortion was a bit lower (0.05%).