A) Your friend's case:
The tuner might have crank up the amp gain and also might have reversed the phase on the sub.
B) Your case:
What he meant is level matching in order to achieve linearity but from what you describe, he has not achieved this.
The sub does more than satisfying those bass heads .
Rather its purpose is to complete the frequency spectrum produce from the CD via the HU.
It adds warmth to the music, more so, on the vocals.
In order for it to sound good, cut off frequency setting, phasing and enclosure size are very important in order for it to "blend" well with the comps and if done correctly, will create a total front stage with great imaging and seperation.