"There is a slight amount of bellying which concerned me at first, but now I just live with it."

Silas, there should be a slight rise in the top - apart from anything else, Martins are built with a radiused top - it's perfectly normal. If you place a straight-edge across the lower (i.e. big) bout behind the bridge, it should not touch the edges of the bout on both sides at the same time. The gap should be less than 1/2" (total of both sides added together). If it's more, it might be worth having it checked by a luthier, could indicate a loose brace or the instrument may be over-humidified.

If the top is flat, or if it's sunken in the centre, it's very likely dehydrated (which is very bad and can lead to cracking). Humidification would be necessary in that case.

Martin Dreads are supplied from the workshop with Lifespan Mediums fitted (13-56). IMHO, anything lighter will not produce sufficient energy to drive the top properly, but that's only my opinion, others may differ and that's fine by me.