Nick, your premise about the bigger sub sounding "sloppy" higher up compared with a smaller sub doesn't really have factual support.
This isn't factual support. But, many people around here have mentioned how when they run their m80's or m60's as large speakers with the sub off that the bass just seems quicker, tighter and more well defined. Many of those guys have Axiom subs. Granted, running no sub will not have as much low end extension as running a sub.
This can be perceived for a variety of reasons and it has nothing to do with the smaller bass drivers being tighter and more well defined.
1) A poorly designed subwoofer.
2) A well designed subwoofer that is not properly calibrated or running "hot".
3) Improper crossover integration between the subwoofer and the main speakers when set to small.
4) Placement issues. Some have found this "boomy", "flabby" characteristic to occur from corner loading a sub for example.
Here is another article on this topic that you might find interesting as well. It is from Alan Lofft of Axiom audio.
http://www.axiomaudio.com/tightbass.html
