I don't have the PX100, just the PX100ii. Although the sportapro and px100ii sound similar, I prefer the sportapro by a fair margin (I found too much of a mid bass hump on the px100ii and some veil and rolled off highs - maybe I had a dud?).
You could get a Koss Sportapro ($20) and clip the drivers onto a Parts Express headband ($2). This gives you "Portapro sound" on a lighter headband that doesn't clamp as much (thus balancing out the sound nicely).
Well, I love the audioquest nighthawks. But others will chime in with their preferences.As far as planars are concerned, these might end up being surprising..http://www.head-fi.org/t/820107/monoprice-monolith-m1060-and-m560-planar-headphones/0_30