4) Finally, I suspect the reason that 555 hasn't taken off as a jam vehicle for the band is the awkwardness of that closing progression: Bbm > A. It doesn't fit any traditional key or mode, so it seems like Trey has a tricky time finding a real melody to put over it. He tends to rely on one bar phrases, then a bend over the tricky change, eliding the transition. It's almost like that song needs to do what Split Open and Melt does rhythmically. SOAM starts with that extra bar of 9/8 every fourth time through, but eventually the band abandons the rhythmic intricacy for an open jam, eventually returning to the original rhythm to signal the end of the jam. (I know it wasn't always like this back in the 90s, but since 2.0, that's basically been how it goes). 555 could do the same: Bbm > A for a while, then settle on one of those chords, and go deep. (The way Simple eventually settles on either F or Bb.) The problem seems to be that the Bbm > A progression doesn't leave enough melodic options open for the band to settle into a space where they can move into another jam in just Bbm. It goes around the progression for 2-3 minutes, then Trey realizes he's out of options, and hard segues into Light or something.

Great comments. It's common in these threads to hear comments such as "....the 'segue' from Sand to Light was jarring..." or something like that. Yet Light, for example, will almost always sound "jarring" coming out of ambience, regardless of key or tonality. The beginning of Light doesn't lend itself to segues. Just as you point out, many songs' endings don't lend themselves to easy transitions either. It's just a lot easier with vehicles like Disease and Ghost since there are so very many options with songs like that which don't offend the sensibilities when employed. Great comment on Panic as well. I hear the same thing and it's prevented me from every really embracing them. I'm spoiled I guess.