SUBWOOFER x OVER

Thanks Kr4,the ins/outs are preamp to amp not speaker.The sub without the highpass filter has no speaker option.

It sounds like to me that the unfiltered high pass is implying that it doesn't implement a crossover for your main speakers. I've found it's best to cross your sub over as low as possible (40hz-60hz) and center your sub in between your speakers. What are the subs your are looking at?

B Limo,The subs are SVS model SB12-NSD and the SB-1000.

So, the high pass is line level RCA. That's good because speaker level would imply a passive iron core inductor. Also on that model, it has no speaker level inputs where the SB-1000 does. That means if you want to use high pass, you need cables to and from, which could be quite a distance and limitation.

Personally, I like the idea of using a high pass on the main speakers, particularly if they're smaller satellites, but this seems kind of awkward. Could be done more directly with a Harrison Labs FMOD and a Y-splitter or a MiniDSP 2X4.

Why I like a high pass is that it sometimes makes integration easier and, often, the mains just aren't that good at bass. Don't want to get too technical. Comes down more to variables and implementation than anything else.

You can make life a lot simpler by going the REL route. Connect it to the speaker terminals of the amp and adjust LP filter below which you want to pass on the signal exclusively to the sub(s), dial in the sub volume and phase setting and no need to fuss over a Xcrossover etc.
All the best