Skip- If your sub only has line level inputs, then yes order the preamp with a mono out. I prefer to run our sub off the amp at speaker level, but it's not a night/day, better/worse situation. What amp and speakers do you have or are thinking of? Happy listening, Chris.

You might want to take a look at the Jensen ISO-MAX line of isolation transformers.

I use a speaker level to balanced model to connect my SUB225s to the output of my SE84UFO25.

I have found this to be the most transparent way to connect a line level only sub to the SE84UFO25.

You would want a speaker level to RCA single ended output model.

I believe connecting to the speaker level output of the SE84UFO25 will maintain better phase than trying to connect to the line out of the CSP325.

That is the way it was explained to me, it might be the wrong explanation, but sounds the best to me.

The Jensen ISO-Max have the added advantage of using isolation transformers that will get rid of any noise. Once I get my amp, I'll be trying the usual options, and compare the noise level with and without the subs, and decide from there if I want to order stereo ISO-Max and which. It would be a pity to use a Decware amp, which are reputed to have no noise issues whatsoever, and feed some into the system via the subwoofer connections. Now in my system, I have multiple options, including tapping off the speaker inputs, but if I didn't, I might consider going the isolation transformer route without trying the alternative. Just my two cents' worth, of course.

Greetings from Switzerland, David.