Been using the 56u for the last couple of days and tbh it seems a bit erratic signal (dBm wise using Inssider).
The 2.4GHz is especially poor, with the 68u in the back room (through 2-3 walls) and put back to stock power outputs it sometimes eclipses the 2.4GHz signal strength of the 56u in the same room and line of sight.
The 56u 2.4GHz is up and down, it can drop 10-15dBm in an instant.
I've scoped the channels in the area and there is nothing on the channels (1 or 11) i am using (i'm only near a couple of other houses).
When in the same room as the 68u the signal for both 2.4 and 5 is no more than 30dBm, with the 56u its around 50dBm.
Seems from a quick Google that there is other reports of particularly poor 2.4GHz.
Using Merlin Fork update 5 on both routers.
Thinking i might return the 56u and get another 68u, slight overkill but it should all be good then.....