2 inputs into single sub - possible?

"You can also sum the L and R outs of the pre-amp together for more gain if needed to the sub."
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For most preamps, this is not possible. If you sum the L & R signals from the preamp using a y-adapter, not only will the signals going to the sub be summed, so will the signals going to the main speakers. For this to work, your preamp needs to have separate buffers for each pair of RCA outs.

If it doesn't, you can use a single Jensen isolation transformer (SUB 1 RR) to isolate the L from the R signals, and then perform the y-adapter summation after the transformer.

You could also use a similar approach for your HT and music scheme. Just run the LFE out to the input of transformer and then sum the output of your music preamp with the output of the transformer using a Y-adapter. In this case, you'd only have a single output from your stereo preamp. To sum the stereo to mono, you'd need 2 of the SUB1RR xformers.

Yes, you can run them into the sub that way at the same time, as long as only one is on. You can also sum the L and R outs of the pre-amp together for more gain if needed to the sub. It will also keep you from loosing any lower bass info that is rarely mixed to just one channel of some stereo recordings... (doesn't happen very often).

Rel subs are designed to accept high level (music) and low level (LFE) inputs simultaneously, but I'm not sure they can connect to 2 different systems for use at diff times. It may work, but I would double check w/Rel.

See response to other similar thread above. There are some brands of subs that can be set up for HT and stereo, like REL.