Again, the Devil is in the details, we would need Intimate information about your specific room and the placement within the room to make any kind of recommendations.
The problem is that most discussion of Dual or Multiple Subs are about Movie Surround Systems. So, it is hard to transfer that discussion to Music systems.
If you have enough space, sufficient space that you have a degree of flexibility in placing the Subs, then perhaps you can gain something.
For myself, in a pure Stereo system, I have often considered Dual Stereo Subs, but rather than one or two large Subs, I would be more inclined to go with two Smaller Subs, probably 10" but perhaps in the right circumstances two 8". Keep in mind while I want deeper bass, for Music, especially for my music, I don't need 20hz or Sub-20hz bass. If I can drag it down smoothly to 25hz with Drivers similar or slightly larger than the drivers on my main Front speakers, I'm good.
In reality, the only thing I can think of that does Sub-30hz in Music would be Pipe Organs, then we have to ask, even if you are listening to a Pipe Organ, how often are they really hitting those lowest of the low notes?
So, if I can go Sub-30hz, fine, but I don't need to go much lower. In my own experience with two speakers having 2x8" each and rated at 28hz, when I played 28hz tones, I did not hear a tone, rather I simply heard the
Fuff-Fuff-Fuff of the driver moving. Again, the point being that while Sub-30hz is nice, I don't need to go all that much below it for the music I commonly listen to.
Also, let's say you use 2x12" Subs in a reasonable sized but still modest room, just the volume of air creates a degree of impact. I would be worried that no matter how low I functionally turned the Subs down, they would still push so much air in the room, that they would stand out from the Front speakers.
So, like I said, probably two 10" would not be grossly bigger than the speakers I'm likely to have, and with the right 8" Sub, perhaps even two 8" would do.
The key is that buy pushing some of the bass weight off onto the Sub, you are making the job of the Front that much easier, and pushing the bass off on to the Sub, I speculate, would likely clear up the Front Speakers nicely.
You can see how much more complex this is for a pure Music system.
In your system, the
Yamaha NS-B750 Bookshelf Speakers are not cheap, but they are a 5" (130mm) 2-way. The
Yamaha SW700 is a 10" (300w, 20hz) Sub, that seems like a very nice blend, and those should compliment each other nicely.
NS-B750 - Speakers - Yamaha - Australia
NS-SW700 - Subwoofers - Yamaha Australia
Sorry to use
Australia, but it was the first reference that came up on a Search.
If by chance you decide to try TWO Subs, I would either get another SW700 or get two New Subs.
Though again, it sounds like you have a very nice blend of the system you have now. I'm not sure it needs improvement.
Curious, there must be some type of
Calibration/Setup program in the RN803, what did it choose for the Sub/Front Crossover? The Calibration/Setup Program actually measures the room and sets things accordingly.
Also, have you run Room EQ with the system?
Steve/bluewizard