I wish I could do distortion tests well enough to post Face. The truth it though, loudspeaker distortion has to be pretty gross for it to be audible with music. That's not saying it can't be, just that it's usually not a problem for any decent driver used in the range it's designed for. For instance, I have a 4 inch FRer that just should ever play anything under 100Hz. It's distortion elements are only 20-30 dBs down and even though they're low order, I believe they're audible.
check out these tests for yourself:http://www.klippel-listeningtest.de/lt/default.html
Remember that this distortion is added to what your headphones or speakers are already putting out. Unfortunately after doing these tests you'll be able to hear distortion more easily.
Practice makes perfect. And you'll understand that it's not typically a problem.
Wushuliu, it's not like they're new or make DIY obsolete. They definitely make DIY look less worth it unless it's for the love of it or you are more brilliant than any DIY guy up to this point that I know of. Those measurements are great by anyone's standards. If you don't believe me, measure your speakers. Just to get the quality of drivers that are in these, you'll spend more let alone all the other parts and not to mention the work.
Nullspace, definitely cone and dome on the second. Unfortunately I don't have a functioning impedance rig so no guarantee for tube amps. They aren't very efficient either so even if the impedance was good I don't know that I'd recommend them for typical tube amps.