Connecting dual subwoofers to a stereo preamp

Thanks @blisshifi. That helps. I had a feeling that would work.

I'm still hemming and hawing about whether to hook up 2 in stereo or not and even whether or not to get 2. I've had one nice sub for a long time and I've never been unhappy with it (my M&K V125).

I'm considering just getting one SB16-ULTRA for now and running it just the way I've been for years. When I have a more flexible budget, if I feel the need, I'll get another SB16-ULTRA.

My 20 year old Nautilus 805's are still amazing along with the rest of my gear (Rotel amp and Adcom preamp) even without a sub but they really do need it and when you're used to it it's a bit of a drag when you have to go without!

@udo If you have the room and budget, dual subs are worth it. Having a single sub will not deliver the clarity and imaging across the whole width of the sound stage. For bass, a single sub may just ever so slightly manipulate where a bass player may be standing on the stage. Not drastically, but sometimes every last measure counts. But my biggest affinity for subs is to only use them for their lowest frequencies, almost inaudible, such that their waves clean up the rest of the frequency spectrum, and having two or more enables this to be done evenly across the room. When I sometimes had one sub in the past, I felt that there was more openness on the side where my sub was placed. My room is 19’x23’ with a sloped ceiling that goes up to 18’, and two SB-4000 work very well. Prior I tried two SB-3000 and they didn't go low enough to make a huge difference. 

@udo 

Have you done any research into implementing a "distributed bass array"? Sometimes it is referred to as a "swarm".

It uses 4 subs, not 2. Pre-out left and right are sent to a crossover unit like the MiniDsp 2x4, where a crossover frequency is applied prior to the distribution to each sub.

With 4 subs most of the peaks and nulls created by your room are mitigated.

The improvement is amazing.

Go dual, you will never regret it as long as you place them correctly. In my experience, do not place them by the mains just because you want stereo. Place them on different walls. One behind the right speaker and one a third of the way down the left wall is my setup. They still play perfect stereo and give you clean bass at any position in the room. Placing them by the speakers results in heavy boomy bass along the back wall, at least in my house of stereo and it is heavily acoustically treated. That was one of the best tip I got on this site. Wish I remember who it was to thank them.

What you’re doing isn’t clear to me, but it looks like you’re combining the left and right channels together at the amplifier output to create a mono signal for your sub. Is that correct? If so, the problem I see is that most amplifier and preamplifier outputs are wired in parallel. So if you tie the L and R outputs together at an amp output, you’re likely to have mono signals at the amp's other outputs, too.