I meant as in the only benefit is reducing seat to seat variation, and that the rest can be done just as well by external EQ that sees both subs as one. And also if the seat to seat variation is small and you have such external EQ, then you probably won't need many filters and hopefully even the 3 bands from SVS will suffice? And basically an extra sub with optimized delay can help flatten the response at the MLP but individual subwoofer EQ can't really do that any better than just EQing both subs as one? Then again MSO also optimizes delay, doesn't it? And for that one would still want flattening and not just seat to seat variance reduction?
Sorry but I am a bit confused about what exactly can achieve what, and what doesn't actually help.
MSO can work in two modes.
Either reduce seat to seat variation, in this mode you would still need global EQ or input EQ to make sure that the response is at the end flat. This mode does not in the slightest focus on a flat response in ANY of the seats. but just that the response is more or less the same in All seats.
Or the general optimization mode which focuses on both reducing seat to seat variation AND achieving a flat response at the same time (or whatever target curve you provide) for all seats.
Your comments about 'What If there is not much seat to seat variation' are mostly academic, which is why i'm finding it difficult to understand why this would be of interest to you.
I'm going to give an example of my own living room. I have taken 5 measurement points over a small couch (180cm X 80cm), there is no EQ but the location of both the couch and my two subs have been EXTREMELY optimized over the years, the delay between the two subwoofers is also ideal.
Even in such a small space, with two subs, and countless hours of optimizing the location of everything there is still a huge seat to seat variation at the ~35Hz room mode (a whooping 20 dB variation). Luckily very little material have energy this low so i found it to be the best compromise for my room.
The only way i could minimize variation at 35Hz at this point is if i buy two more subs, which is what i'm planning to do in the future.
Most people have several seats that spread over several dimensions (unlike my small couch) and would exhibit much more variation with two subwoofers than I do.
By the way your comment about an extra sub inherently flattening the response is generally incorrect. it will only reduce seat to seat variation and perhaps filling up deep holes in the response. but the peaks remain untouched.