Adding 6" of cellulose to the attic will help tremendously, so long as
the soffit vents are kept open with some baffles. You'll add an
average R value of 3.5 per inch of the cellulose so you'll increase
your attic insulation from R-19 to R-40.

Other things you can do is move around your house with an incense stick
looking for drafts. Seal up as many drafts and leaks as you can.
There are professional services that will install a temporary blower
into a doorway and will actually pull air from the house, creating a
slight vacuum. Then they will actually test how much negative pressure
is created and how much air volume is moving through the fan and from
that determine how "leaky" your house is. They usually go around with
a smoke pencil looking for leaks while this is happening too.