Please help troubleshoot/repair REL Storm III

Hi guys,

I was just gifted a REL Storm III which I will use as a pair along with the one Storm III that I already have in my system. Please help me diagnose the problem with this new sub. I have very rudimentary knowledge of electronics. I am good with a soldering iron, can identify the components on a PCB, can use an ohm meter, but no more than that. I am hoping that I can be steered in the right direction and that the repair might be as simple as replacing a malfunctioning part. The problem:

All the functions and controls seem to work properly, but compared to my other Storm this sub‘s output is much much lower. In order to get equal volume from both subs at normal listening levels, this new sub’s volume control needs to be almost maxed out; while the good sub‘s volume control is set at approximately one third in its rotation. I use the high level inputs and connections to my tube mono blocs has been triple checked and are correct. I have swapped the subs to opposite channels with the same results; the low output follows the new sub.

Thoughts, suggestions? Thanks very much; it is appreciated.