I've heard some truly awful, thin/boxy sounding 12's and some thunderous, deep 12's (inc' in that are 2x12's as well). Likewise I've heard flabby 15's and 15's that could shatter glass... and as for TENs!
I have to agree that it is pointless generalising because there are as many exceptions to the rule (10's toppy, 15's bottom, 12's middy) as there are obey the rule. It's good fun trying them all out though to come to this conclusion.