Are you sure it's your seat? I don't mean to be condescending or anything- but how is the flexibility in your lower back and hamstrings? The only time I have problems is when I haven't been in one of my kayaks for quite a while and I need to start stretching again. Usually in January when the pool sessions start and I haven't been in a WW kayak in at least 3 months or the first couple paddles in the sea kayak in the spring. I also find that if I move my legs around in the kayak, for leaned turns and rolling even if they're just for fun, or when moving my knees for my normal forward stroke in my sea kayak the numbness doesn't come or goes away.

If your flexibility is bad and you need a workaround a new seat may be just the ticket. Flexibility is good though, too.

m