701 has a great design for stacking, and this isn't to take away from an excellent robot design fro balancing. However, I must disagree with you on 1501's method of balancing.
Based on the footage I've seen (here's a video of their triple balance technique:
http://www.youtube.com/watch?v=KVpnR...ayer_embedded#!) It would appear that 1501 was capable of balancing with 4028 on top of it without another robot. I reached this conclusion via observing the lack of significant movement of 4028 on top of 1501.
This isn't to detract from your balance, which does indeed appear to be the first double balance of its kind. I'm just trying to point out that 1501 appeared to be no less capable of pulling off this feat.