Amp level versus line level inputs for subwoofers

I use the speaker level inputs on a stereo pair of subs with great results both with music and movies. My subs have RCA, XLR and speaker level inputs.

I assume that if the signal comes from one amp or mono amps, the sub would then retain more of the sonic characteristic of the main amplifier and less from its own internal amplifier

I agree absolutely. Mine are not connected to the amp, but to the speaker binding post via bananas.

Line level comes from the amp output (or from the speaker) but is a very high impedance (on the sub) which means it is sensing the voltage and not using any real current to drive it. The best scenario (in my opinion) would be a sub out from the pre with a built in (or external that would go between the pre and the amp if not built in) crossover. That way there would be more available current for the speakers since that power won't be wasted on something the speaker can't reproduce audibly. Without a built in or external crossover that current is still going to the speakers. So if a crossover is not possible I would prefer line level.

Hope that helps

The interface between amp and main speaker is highly complex, and the signal that is present at that point reflects not only the sonic character of the amp itself (and what precedes it), but effects that depend on the speaker as well.

Those effects include phase differences between voltage and current, which depend on the degree to which the speaker's impedance is capacitive or inductive (as opposed to purely resistive), and also back-emf (voltage produced by a driver within the speaker due to its motion continuing after the signal has stopped or changed). All of those effects are highly dependent on frequency.

Since as Xti16 indicated the sub's amplifier only senses voltage and not current, its response to those effects will be partial, and will be essentially unpredictable. Whether the sonic result of that response will be for the better or for the worse therefore figures to be system-dependent, as Raquel stated.

I assume that if the signal comes from one amp or mono amps, the sub would then retain more of the sonic characteristic of the main amplifier and less from its own internal amplifier.

More of the sonic character of the main amp -- yes.
Less of the sonic character of its own internal amp -- no. I would expect the sub's internal amp to be just about entirely in the signal path to the sub's driver whether the input is taken at line-level or speaker-level. If it is taken at speaker-level, I would expect that the incoming voltage would either have a lower gain applied to it at the front end of that signal path, or else that it would simply be divided down to a line-level amplitude and then processed identically to a line-level input.

Regards,
-- Al

Look at the Vandersteen web site for info on how the 2Wq subwoofer works. It is very much like what Xti 16 posted, and addresses some of the phase issues Al mentioned. I have had a pair of these for a few years now, with two different pairs of speakers. IME, these offer value/performance ratio that is off the charts. Bass is extended, powerful, tight, tuneful. The subs do not make their presence known, and blend seemlessly with the mains. Perhaps best of all, they are designed for corner placement. In my room, they work perfectly in the front corners with no acoustical bass treatment devices and no external equalization devices other than the in-line high-pass filters.