First of all, you might not want to use Audyssey, period. I wouldn't use Audyssey. It's a bandaid for bad speakers, and you don't have bad speakers. Your speakers do not need auto-equalization since I am pretty certain they are tonally balanced to begin with. If you want to change their sound, use a graphic equalizer or parametric equalizer or tone control. Audyssey can't do much to help a good system, but it can hurt a good system. The only good it can do is taper down room related peaks in bass, and that is only if you take the measurements correctly, and even then its not worth running if you can't contain it to low frequencies only.
Regarding the subs that you are considering, in terms of performance, what you are paying more for is deep bass capability, mainly stuff between 16 Hz and 25 Hz. Above 40 Hz, what you get is almost an inverse of the sub 30 Hz capability. Let's look at the 16 Hz burst output from some of these subs. I am only going to list measurements from their lower tuning point, since that will put their best foot forward in a 16 Hz test:
- Outlaw X13, 1 port: 102.3
- SVS PB-4000, 2 ports: 107.1
- Monolith 15" THX, 2 ports: 105.2
- Hsu VTF-3 mk5, 1 port: 99
- SVS PB16-Ultra, 2 ports: 109.1
- Hsu VTF15h MK2, 1 port: 102.3
For some perspective, consider that in the dB scale here, a 6 dB increase is a doubling of output, a 3dB increase is about a 50% increase, and a 9 dB advantage is about a tripling of output.
Now let's look at their 25 Hz performance, this time in the higher tuning modes:
- Outlaw X13, 2 port: 111.1
- SVS PB-4000, 3 ports: 113.3
- Monolith 15" THX, 3 ports: 113.1
- Hsu VTF-3 mk5, 1 port: 110.4
- SVS PB16-Ultra, 3 ports: 116.9
- Hsu VTF15h MK2, 2 port: 111.8
Now let's look at how they do in mid bass, we will go with 63 Hz since that maybe the most representative for this range. We will look at the lower tuning modes:
- Outlaw X13, 1 port: 114.1
- SVS PB-4000, 2 ports: 115.4
- Monolith 15" THX, 2 ports: 115.8
- Hsu VTF-3 mk5, 1 port: 116.8
- SVS PB16-Ultra, 2 ports: 117.1
- Hsu VTF15h MK2, 1 port: 116.9
You can see, they are all pretty close at 63 Hz.
The point here is that, between these subs, you are basically paying for more powerful deep bass, from a performance perspective. But there is more performance than just easily comparable burst numbers, and there is more to subs than just performance. There is also stuff like nicer finishes, longer standard warranties, features like SVS's app control, shipping practices, and the logistics of dealing with large sizes and weight (don't dismiss this so readily- you will regret it otherwise).
Something else to consider is that two subs will give you a more balanced response and less chances of localizing the sub, so basically two subs can grant you better sound quality- if you take the time to set them up properly.
