You mean the one were you said "
If there is an issue shown in the waterfall it will usually show up in the group delay chart or vice versa"? If so, that's what actually prompted my original post.
The 'usually will show up' part is a bit of a concern, but the biggest issue I guess is the waterfall is easier for me to read/decipher. That, along with the distortion and compression tests, are usually the first ones I look at, because combined I think they give a good picture of the basic functionality of the sub.