That does make it seem a bit like a sealing issue if the bass stayed low with increase volume. If you ever want to do a pretty good volume match, a multimeter with AC voltage is a good option for cheap.

Do you have access to a PC to listen with that you can use EQ APO on? If so, you might put a low shelf on the one that has less bass. If it takes more than 1-2dB to sound like the other one then it is probably certainly a sealing issue.

An alternative test is to use a tone generator like this one and see how low it goes. If the seal is broken on the less bass one, it should roll off much sooner than the other.