Connecting dual subwoofers to a stereo preamp

@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.

Thanks for all the input everyone. I think I'm going to go with 1 sub for now. I'd like to use 2 but I want to spend some decent money on them and right now, while renovating my new house (well, old house, new to me) my budget is too tight and I'd rather spend more than a grand per sub. I think I'll one the SVS SB16-ULTRA for now like I mentioned above and then in several months when things have settled down I can get another. 

@cleeds - no, that's not correct. My schematic doesn't show that. I'm not combining the L/R at the preamp. I'm sending L to one sub and R to the other. But I'm not going to do that now. I'll revisit all this later. With just one sub for now, the connections are a no brainer. Just what I've been doing for the last 25 years with my M&K.

@hifibliss - my room isn't as big as yours. It's only 14' x 17'. It's an okay size but I think 4 subs would be too much and like I said above, it's not in my budget. Ideally after I have 2 nice subs I'll upgrade my mains to a full range pair. I used to have a pair of Nautilus 803's but had to sell them 20 years ago for financial reasons. I really miss them. I have to do some research when I get new mains but I've wanted pair of B&W 802D's lately or maybe 803D's. We'll see when I get to that point.

Thanks again everyone!