I have always added my raisins to the secondary carboy, and leave it sit for ten days. At that point, the airlock will stop bubbling. I then proceed from there. I know Joe---and some others---put their right into the primary. Just a different way of doing the same sort of thing, I guess. This muscato is definately not a tart wine, but mine has always turned out very nice. I have not checked the pH or TA, though, so I can't help you there. I'm a taster, not a tester.
Like I've said, it mixes very well with the DB, which is very acidic, so it may be a bit flat by nature. Are you adverse to bumping up the TA?