I've been conducting seasonal adjustment using both X-13 ARIMA-SEATs as well as TRAMO-SEATS for different kind of variables (stock and flows) such as monetary aggregates and balance of payments. Specifically, in the Current Account statistic an indirect approach is followed, by seasonal adjusting its components, and summing them up to build the seasonal adjustment current account. The main problem that I have is that some of the components do not exhibit seasonality, then I'm wondering if the correct approach is to sum seasonal adjusted series with non-seasonal series without adjustment, or with adjustment.

Thanks a lot, and if you could provide some literature related I'd be very grateful.