I agree with almost everything you posted. I mainly agree with your 3rd point but would just add that any achieved overall system performance gains, made from high passing your main speakers, are dependent on the degree to which your main amp(s) and main speakers benefit by being relieved of deep bass duties. The higher the main amp's power and the higher the bass extension limit of the main speakers, the smaller I believe the overall system sound quality performance gains will be.
Hello dancub,
Adding a 2nd properly positioned and configured SVS SB-1000 sub to your system, in my opinion, would result in the bass being perceived as sounding roughly twice as good as a single sub even in your relatively small room. You'll notice a general sense of ease to the bass, an increase in maximum bass output and an increase in the realism of bass dynamics due to the total bass duties being shared between two subs
neither of which is operating near its limits and both having ample reserve power for reproducing the powerful bass dynamics of music heard live at a small venue.
You'll also notice the psychoacoustic benefits to the sound that result from the use of multiple subs in any given room, that come into play beginning with two subs producing bass in a room and that are further enhance perceived bass quality in degrees up to a practical limit of four subs. The clearly noticed progressive bass quality improvements noticed with multiple subs in any given room are an increase in perceived bass detail, speed, smoothness and improved blending or integration between the bass reproduced by the sub(s) and the midrange to treble reproduced by the main speakers. The overall sound will sound progressively more cohesive as subs are increased from one to four in degrees.
I know the SVS SB-1000 subs are small in size but very good performers that are a steal at the current price of $499 each. My opinion is that buying and trying a 2nd one is a low risk and bargain price to experience bass response performance that I'm confident you will consider approaching state of the art even in a smaller room such as yours. Worst case is you try it out for 30 days, don't notice the improvements and return it for a full refund. Best case is you try it out, discover you suddenly have exceptionally good bass response in your room/system and begin thinking of the further gains possible by adding a 3rd or the ultimate fourth sub. Other important qualities of your sub is that it's relatively small, has all the important configuration controls and overall is an ideal sub to be used in multiple sub systems.
I should also mention it's very important to precisely position and configure each sub once you enter the wonderful world of multiple subs. I have a lot of useful experience and knowledge positioning and configuring 1-4 sub bass systems gained through research and using them in my own system as well as setting the up for friends' and relatives' rooms/systems and I enjoy spreading the word on how well the concept works and assisting setting them up.
Of course, the decision is yours but I'm also willing to help you out with the optimum positioning and configuration if you'd like (I'm retired and have the interest and time). But I'm also sure SVS's call in support could also assist with this if you'd prefer.