I think you're right, I do spin on the slow side. 70-90 is more my relaxed preferred rpm, 90-100 and higher when I am putting the hammer down.
Some time ago I finally realised that pedaling is about 2 things - spinning and torque. Your legs are comfortable putting out a certain torque. If you're riding slowly, then there is no point in spinning for it's own sake - use your preferred torque and just spin slower, your knees won't complain any more than when using that same torque at higher speeds. If you're going fast, then that same torque at a much higher rpm, results in larger power. Chris Carmichael said the same thing about LA - Lance isn't spinning fast to produce large power, he can spin fast because he can produce large power - exactly the other way round.
Now at about 40km/h I spin comfortably and I can go quite a bit faster, eg 50 is not a problem; but then I need to concentrate hard to avoid bouncing on the elastomer, and of course it tires you out quite soon. I prefer to relax a bit more.