Thanks, dgluke, for the John Atnkinson Stereophile link. The whole article is interesting and informative. In any case, I did not know about "B weighting"; my SPL meter only gives me a choice between "A" and "C". There are, of course, several other equalization curves in use; "A" and "C" are just the most common. But, as Atkinson remarks at the beginning of the article, "A weighting" does not correspond to perceived loudness levels, which is what I've certainly experienced and pointed out above—and the SPL meters mentioned, and even linked, elsewhere in this thread are all to "A weighted" devices.
As for the relevance of using an SPL meter to judge how low a speaker's frequency response might go, let me quote Atkinson's own words from that same article: "If you plot, say, a loudspeaker's sound-pressure level against frequency for a given input voltage, the result is the typical amplitude or 'frequency' response."
In short, using an SPL meter in conjunction with a low-frequency test track is, indeed, a valid way of determining "how many [H]ertz my speakers measure for bass," in the OP's words.