anyone have? car stereo tuning cd

[taank]

Sub amp only . 8 inch svc 4 ohm jl w3
Amp is a jl xd 600/1 rated at 400 at 4 ohms
I turned the "gain" up till I hear distortion . Problem is
It never distorts the sub rms is 250

[Akromix]

Sub amp only . 8 inch svc 4 ohm jl w3
Amp is a jl xd 600/1 rated at 400 at 4 ohms
I turned the "gain" up till I hear distortion . Problem is
It never distorts the sub rms is 250

Either you can't hear distortion from the sub, don't know what it sounds like, or the amp is not being driven by a very strong signal.