Hello!  We moved into a new house and are looking to remodel the attic.  In one section of the attic, the floor joists are 2×8 SPF #2 16″ oc running a 15’6″ span.  The previous owners stacked 2x6s on the 2x8s for about 8′ lengths starting about 4″ from one end of the 15’6″ span.  They put perpendicular 2x6s on the ends to create a box and attached 3/4″ plywood to the top of the box.  The stacked 2x6s are nailed into the 2x8s.  My remodel plans include only the area currently covered by plywood.  Should I leave this base as it is or add 2x8s for the full span to reinforce the floor?  If I add the 2x8s, do I remove the stacked 2x6s or stack new 2x6s (I’m not sure if I should sister the 2x8s or center between the existing 2x8s)?  The goal for the space would be an office – walls, desk (lightweight), desk chair, and a couple of other chairs – nothing particularly heavy.