I just got a SVS 20-39 PC+ for my 14'X18' HT

. I had to reduce the Subs sound level to -6 db on my receiver, and the PC+ volume to between 1/4 and 1/3. This setting makes my Sofa tremble at high Bass scenes, e.g. LOTR ... etc

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Also, the sub is HUGE. Like you I was thinking about getting a pair of these monsters. Now I am glad that I just got one. They are brilliant subs, but placing two of those in my small room would have been difficult.
My recommendation, I am not an expert, is to go with one. They are HUGE and unbelievably powerful. You can add one later if you need to.
When I contacted Tom from SVS about my Sub purchase, he recommended a PCi for my room size and listening habits. I went with the PC+ and it is excellent. You can always contact the guys at SVS with your questions, listening habits, equipment, and room layout. They will give you a recommendation in no time. You can contact them at
[email protected]
Good luck. These are excellent beasts. You will be surprised at how your system sounds after fitting one of these monsters.