Just a word of caution and a suggestion to email Almarg through A/gon mailer and see what he has to say on this type of set up ...
What I've gained from his posts and comments is ... if you try to split the preamp out signal to both the sub's plate amp and the main speaker's amp simultaneously ... the preamp will see the input impedances of both the Sub amp and the Main amp
Here's where I forgot exactly what happens and why I ask you to contact him for clarification ... by splitting the signal from the preamp it either reduce the input impedance as seen by the preamp to the lower of the two amp's input impedance's or it reduces the input impedance as seen by the preamp in half ... I forget which one it does
Sub amps usually have a very low input impedances and can either drag down the main amp's impedance that the preamp sees or reduce by half the impedances that the preamp sees
If the subwoofer's plate amp has a 12K ohm input impedance and the main amp has a 47K ohm input impedance ... they can either both be reduced by half or be seen as the lower of the two reducing the main amp's input impedance to 12K ohm
This may make it difficult for the preamp to drive both into these lower amp input impedance's ...
I looked at the SVS manual listed and it doesn't give the spec for their plate amp's input impedance ... but I would not be surprised if it is about 12K ohm
The same thing can occur with a two channel preamp that has two separate outputs that are not buffered from each other
A way around this would be to use a buffer to the main amp so it's input impedance is protected from the much lower sub plate amp's low impedance shifting everything around
In HT the processor or AVR's Sub Out connection by design are usually buffered from it's Main outs as in HT it is expected that your main amp and sub's plate amp may has a wide difference in input impedances
In a stereo two channel preamp with two main outs ... I think they were designed to drive separate mono blocks having the same input impedances .. so it's not a problem
Just a word of caution an a referral to a higher more knowledgeable authority
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