At Location A - the room directly above the router - the RT2600ac delivers solid results on both bands and leads the way in downstream performance.
Location B - across the ground floor at a distance of roughly 45ft - poses more of a challenge, and all of the routers on test reveal their limitations at this sort of range. Synology's RT2600ac manages to maintain decent-enough speeds, and edges out its RT1900ac sibling, yet it's in this situation that a mesh network with multiple access points may deliver the best results.
Location C, on the top floor, also returns solid results. The RT2600ac stands out on the 2.4GHz band and delivers the highest throughput on 5GHz, too.