Identifying old mtx sub

[audioholic 5,000+ posts]

I paid 30 .it sounds good on 300 watt amp.

I'm not using these as a main but just something to hit lower notes that my 4x10in subs cant

Mixing sub sizes, unless you really really know what you are doing, is usually a bad idea. And almost always the worst way to fix a problem with frequency response. To do it properly it would require significant processing, a lot of testing, and getting a bit lucky. Four 10's is plenty of cone area to get solid output down to 20hz, unless the enclosure design doesn't allow them to, or enclosure location/firing direction is causing problems.. Check your box alignment and position before just tossing a 15 in the mix too. Id suggest either altering the four 10" setup you have now, or replace it entirely with a different setup that has the frequency response you are looking for.