Raise concrete floor with carpet to same height as adjoining floor
If you choose to use plywood, go with the 3/4" T&amp;G for the layer under the carpet. The trough were the heat pipes used to be can be spanned by the 3/4" plywood too. Also since the carpet compresses under foot, keep the plywood below the hardwood layer about 3/8" or so lower. That way when you step there, the transition will feel level.