I use SPD clipless pedals and I had the same problem (numb toes and even some pain) once my rides started to get longer. My solution was to use some insoles, since I use them for running, too. Specialized has some specific for cycling. Also, I have tried to wiggle my toes periodically. I have had only one, mild, incident of numb toes this season, towards the end of a century ride. With the clipless pedals, save a major biomechanical issue, I think it is simply easy to keep putting pressure and not move our feet enough.

Since you use power grips, I wonder if you use cycling shoes, with a stiff sole. Perhaps the flexibility of the shoes you are using is creating pressure that takes a toll over a long ride. I have friends who use MTB shoes even though they do not use clipless pedals. And, since you usually ride "sockless", be wary of socks that do further restrict your toes...