I'm not sure if your asking if there is a subtitute way to measuring SPL or if there is a substitute to the specific radioshack meter.
In order to set the output level the same, you have to measure the output level- the only wat to do that is a SPL meter. If you're thinking of a substitute method to get a quantitive measure of Sound Pressue, I'd be curious as to what you had in mind.
However, You can get ones other than the Radio Shack meter (there are dozens of models out there)-- but getting a RS meter should be super easy as they are carried in all RS locations, and there is usually one within a 15 minute drive of anywhere in the US!
That is an interesting idea. Another problem: It has been many years since anyone manufactured a cassette deck with mic inputs. An older deck with analog meters (we’re probably talking no later than the early 80s here) would do the trick, using the 0dB point on the scale as a reference.
Calibrating the sub would be the trick, however. Typically budget mics like you would use for this do not get good low frequency response.
Actually, I was only daydreaming. I'm sure you're right about these decks being old. Over my right shoulder as I write this is my deck with two meters. I bought it in the mid-1970s! The meters are called "VU" meters. I don't remember what VU stands for.