Diagnosing inadequate bass in an audio system.

This is a very broad question.  I'd say you need to be more specific. :)

Try this.

Excellent question which I think most don't understand.  

So do you want accurate bass or the exaggerated bass that is so often provided in cheap sound systems, car audio, etc.  

Most high end (accurate) systems will be judged to have inferior bass compared to a lot of consumer systems.

Jerry

There are test discs with consecutive tracks with different test tone frequencies that go all the way down to 20Hz.  Get one and see where you start to detect a falloff in output.  More scientifically, you could see how they do with a SPL meter.  All that being said, yes, a ruler flat response all the way down to 20Hz, while also being very, very unlikely, may also not sound "realistic", whatever that means.

Get a loudness control.