I posted a similar thread about 2 months ago, or maybe I just posted it in a thread. I, too, had my first noise complaint. I'd been really careful with the noise since moving in. For about 2 months, though, the apartment adjacent to mine was empty. There's one on the other side, but it isn't directly bordering me, I think, or not in the same way. Anyway, I had been a bit louder for some time. I didn't even know people moved in next door until I got a noise complaint. They complained on the first night they moved in. Multiple times, apparently. That was kind of shitty, because the guy who lived next door to me before sort of had an understanding with me -- anytime I was too loud he'd come let me know and I'd turn it off.
Since that first complaint, I've gotten 2 more. Both, fortunately, were the type where the place sends a security guard to check it out instead of just emailing me crudely the next morning. Both times the security guard knocked after standing outside for 10 minutes to tell me 'listen i got a noise complaint...but i don't hear anything. sorry to bother you.'
as you hypothesized, it was the bass for me too. They must've just been hearing the lows somehow. I feel bad but...why shouldn't I be able to play music loud enough so that I can hear it through my whole apartment? I have the bass on my speakers turned literally all the way down, but there's still some. fuck it.