Sorry it sounded like you already have a Sentell in your Musicmaster bass that you were comparing output levels with? Or I misunderstood? That’s the pickup I was asking about magnets in. Musicmaster pickups are traditionally Stratocaster pickups with 6 alnico poles, while a Mustang pickup traditionally has just 4 alnico poles, so if you were using two pickups with identical alnico magnets I’d expect the Musicmaster to be a bit hotter due to the extra magnets. You should get a bit of boost from the series connection of the coils of the Mustang pickup, but they may have just used relatively weak magnets in your Mustang pickup. Lots of different factors involved, but in general I would expect a bit of a hotter wound Mustang pickup to match fairly well with a typical Musicmaster pickup assuming both are using alnico 5 magnets. If your Sentell Musicmaster pickup has ceramic bar magnets then it will probably be higher output than most alnico Mustang pickups.
An easy thing to try is to add some additional ceramic (or even small neodymium) magnets to the bottom of your current Mustang pickup, which will boost its output. You may well affect the magnetism of the current magnets in the process however