Been having this issue for a while, hoping someone can shed some light. My system is an older alpine HU (CDA-9853), 2V pre-outs to an amp driving my front components (4x60W RMS bridged to 2 channels) and a separate set to the amp driving my sub (500W RMS). 2 GA power wire (because welding wire is cheap); all connections including ground are rock solid. I use the aux input with a 3.5 mm jack on my phone most times. Spent a ton of time on cross-overs, EQ and time delay with frequency sweeps to get everything sounding nice. I set gains by ear to ensure sub volume and component volume scale together well (maybe this is my problem?). Pretty standard set-up.

Here's the issue. Volume adjustment on the HU goes from 0-35. Volume seems to max out at 24, so going anything higher than 24 doesn't increase volume any more - both my components and sub see the exact same thing, so I know it's not an amp problem. I can make it max out at 18 or 30 by changing the gains, doesn't change the issue, so gains don't seem to be the problem to me, but maybe I'm wrong. The issue is most noticeable when I play certain songs that are recorded at lower volume levels; when I play pink floyd the max volume is noticeably lower then if I play some chillstep or EDM. Regardless, under no circumstances am I able to get any level of noticeable distortion, so I'm pretty confident the amps are not simply hitting their limit, I think something else is going on.

Any ideas on what's going on? This one has had me stumped for a long time.