Line level or speaker level, scientific reasons? ?

It really depends a lot on your brand of subwoofer. If a REL ST series subwoofer is used for 2 channel music; then the high pass speaker connection is by far the best way to go.

There is no scientific explanation. This is how REL designed it to work, with the main speakers playing the normal full frequency range. They only recommend the pre-out connection for home theater applications because of the AC3/DTS bass signal.

You can read all about it at this link.

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My less than 2 cents is, I've read several comments and reviews that say using the high level input from the amp makes for a more coherent sound. In other words, the sub amp borrows the characteristics of the main amp. Using a separate pre-out line to the sub is said to create bass that can be musically disjointed from the main speakers.

The input impedance on the REL’s(and other quality self-powered subs) is supposedly such that the additional high-level connections to the sub do not affect the main amp.

Using speaker level to the sub then to the mains is an extra drain on the amp and the filters may keep things from blending well.

I like using line level better. This allows me to use all 17 watts to my mains and a good quality sub/sub amp combo. I honestly use a Janus Interphase 1/A sub amp/crossover. This allows for virtually transparent crossover at 100hz, loads of power to the sub(pair of nelson reeds) and never interferes with the amp or pre except for being in between them.

Just an FYI...what Elevick describes is NOT how a REL is setup or how it works. It is parallel to the main speakers and is self powered. It takes nothing from the amp, but the signal; just like the sub getting the signal from a second pre-out.

Yeah my experience with Rel's is that they do seem to work best in parallel from the speaker level connections off the amp(speakers, whatever). Hey, if it works, don't mess with it, ey?!!