Re: Sub question
If it's a self powered sub, then your question is even more confusing. One must assume the mfg chose a suitable amplifier for the driver.
Your receiver/processor amplifiers don't "see" the subwoofer driver's impedance through the LFE/Sub preout connection. Even if you use the speaker level connections on the sub's plate amp (if available), those inputs are really high-to-low level converters that present some ridiculously high impedance to the source amplifier to the point of being a negligible power draw.
Why so stingy with the details? More info upfront gets better a quicker answers.