XV 90 furnace with dual fuel (gas + heat pump). First thing in the AM when heating up after night setback, stat shows "aux heat", but system is very slow to reheat the house when the outside temp is low 30's when the heat pump should be running. Suction line is hot but furnace does not seem to be putting out much heat. Switching to emergency heat warms up the house.

In the low 30's a heat pump does not put out much heat to start with and in almost all circumstances the balance point has been met. You (if capable) or a local on site service person needs to set the system up to kick on the furnace and turn off the heat pump so that the heat is automatic and you do not have to manually turn on the emergency heat switch.

Does the "aux heat" on the tstat mean that the unit has switched over to gas? Dual fuel kit is TAYPLUS103A.

Where can I find a manual?

Yes usually aux heat would be the furnace instead of the heat pump.