What I learned from the auditioning the amps at Karl's was that the more demanding the music, the louder you play the music, or the harder the speakers are to drive (i.e. less efficient), the more the upgrade pays off. Karl plays more complicated music than I typically do, and the difference was more significant on his CDs than on the ones I brought over. However, on my maggies, which are very hard to drive, the bass is tighter and stronger after the extreme upgrade (older Stereo Stratos, green boards), even on my less demanding music.
As for "smoother", I think some passages are a bit smoother - especially if they are on complicated tracks. I have used a couple of tracks that can be hard on the ears to test for this, such as some from Stacey Kent. I think the very high notes are a tad smoother and more refined, with a tad more detail, especially at elevated volume levels. However, this is very subjective, and it is not a major difference, so don't expect to be "blown away" by a difference in smoothness.
From my limited experience, all of the little upgrades start to work together synergestically to take the sound to the next level. I think this is what lures so many of us into this hobby/affliction. I was just noticing tonight how much better my system sounds after adding the extreme upgrade, Groneberg ICs, and the Tempest. I think the Tempest and the Groneberg cables are getting more detail to the Stratos, which creates more need for the extreme upgrade.
