HELP Subwoofer connection, high level or low leve

Sugar... The sub is doing no more "double duty" than any driver is in any system. Think of a 2-way speaker where the mid range driver is playing from 40hz or so up to the point is crosses over to the tweater. The 40-50hz bass does not muddy up the 200hz mid-range provided we're talking about a properly designed speaker. If this were the case all speakers would sound like junk. Right?

here is a reply from REL tech support to a few of my questions:
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The variable roll off control applies to both high level and low level inputs, you would have to manually change it if necessary. The new Stentor III does accommodate this situation as it is possible with the "MODE SWITCH" to bypass the roll off for the low level input, the roll off for this input can then be set via the processor/amplifier providing it has this facility. The high level input is still controlled by the subs roll off control.
Both inputs on the Stentor are always active, this is a benefit when in hone cinema mode as the sub will underpin the main front speakers via the high level input and at the same time reproduce the .1 effects channel via the low level connection.
As you point out for two channel use there will not be any output from the .1 effects channel provided the amplifier/processor is set up correctly.
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from the horse's mouth...

i believe the sub "sniffs" one input first then switches to the other input if no signal was present in the first input. i just dont know which input the sub checks first, the speaker level input or the low level input.

Rap....You may be correct, but it does not make sense to me.
In an Dolby or DTS HT situation basically the ".1" in the "5.1" is the bass. The sub is suppose to play this. Now if the sub is doing double duty by playing the ".1" channel and playing the bass of the main speaker at the same time, I would think it would confuse the sub and the result would not be clear detailed bass, but a bunch of mud.

I agree with sugar mostly. The separate (crossover) controls are for the "III" units only. If you have both types of inputs connected as described during HT the low level input gets the .1 signal from the pre/pro AND the high level inputs are alive as well and will reinforce the left/right mains. This is a good thing for HT.

For two channel there will (should) be no signal on the low level input so all is well. Just the main speakers are reinforced as you have dialed in.

The high level is best for a 2 channel audio system. You may actually have no choice depending on your preamp. The sub is therefore getting the same bass signal as your main speakers, which makes it much easier to get good integration of the sub with the rest of the system.

The low level is best for a multichannel system, including AC-3 or DTS Home Theater. Again, the sub-out on a HT decoder is going to be a low-level connection, so you really have no choice if you want the true Dolby Digital mix.

With the REL you can actually do both at the same time. If no subwoofer signal is received from the HT decoder or preamp, the sub will automatically use the high-level connections. There are separate settings for the high and low level connections, so you can set both to their respective best setting and forget it.

I believe this is all explained in the owners manual.