What should the input for a subwoofer look like ?

[MathWizard]

I'm repairing/rebuilding a stereo, whose SMD PCB I messed up, and it's full of shorts now. So I'm re-making the pre-amp/op-amp section, with through-hole parts.

The new op-amps, of the right type, didn't really work at DC, let alone AC, and I swear I damaged 1 already on this new circuit, and there's no external path to gnd.

So I tried some other's like a jrc4565, and they seem just fine for a rear speaker once I stopped some oscillation. Sine wave in, sine wave out.

But for the subwoofer section, I'm getting more U shapes, than a sinewave, especially at 50Hz-1kHz, but it never goes way. The voltage swing is well within the rails, this is just a pre-amp, so there's not much gain, just filtering, and this is unloaded testing, )Adding some capacitance to the I/P network, I can get sinewave in, pretty close to sine wave out again.

But I've never seen what a subwoofer does, should I be trying to get a sinewave in. perfect sinewave out ?

[Manul]

I never looked subwoofer signals, nor I'm an audio guy, but I would expect nothing less then a perfect sine. I mean, why it should not be a sine at preamp stage? At power stage it is likely a class D stage, so some sort of PWM with filters which might be better or worse, but the input to that should be a clean sine. Distortions at low frequencies would be very annoying for the ears.