If I understand the question correctly, I tested the door tweeters at 4 ohms and the door woofers at 8 ohms. If this amp doesn't have a
"smart" power supply, but a typical power supply with fixed voltage rails, then into a 4 ohm speaker the amp should be "trying" to double the current delivered. Whether or not this presents any challenge to the amp, I can't say.

Most aftermarket amps are rated to run into lower impedances since the designer doesn't know how the amp will be used, but the OEM amps are usually intended for one specific application and so the designer may or may not build in that flexibility (probably not if it adds cost to the bill of materials).

Some amps run into 1/2 their intended impedance all day long... but some then change their distortion characteristics and their damping capability, changing their sound quality as a result.

The DSP subs are dual voice coil so I'm not certain if they are running off of a single channel or not.

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