Cool deal! Glad you got fixed up.
My Miata "bad stereo" experience was at least cut and dry.. no secondary amp..switch pins a-b and cross to c to deal with so it was an easy determination.
If your HU has hi power output.. (my clarion is 200 watts).. you would have found about halfway up the HU volume scale was all you get anyways. Beyond 24 (max of 40) the only thing getting louder is the thump in the trunk from the bass cannon (rca sub out to built in subwoofer amp).

Oh.. and if you haven't broke out the OEM subs to their own channel.. put it on your list of things to do.. trust me here.. LOL