The only way to demolish an OS is to run over it with a Mac truck. A pair of Orbit Shifters's or Cap S2's is the next level in subwoofer performance when comparing to a pair of Subm HP's. Once an individual has more than enough bass with limitless headroom then it truly is downhill after that. Once you hit Max then adding extra is wasted and if one feels more is better than they did not have enough to begin with.
DIY projects are done for many reasons and one big reason is saving money. Carp clearly states that he is selling one Subm HP to get funds to build 4 DIY subs. N8Dogg states he can build 4 21" DIY subs for the price of one OS. They are both correct and if that is what they want and how they want to do it great I really am excited to see it all play out. That is definately one way to get great bass at a good value and pick up alot of valueable knowledge.
I am on the other side of the fence on this meaning for me and my HT room, (which is countryboy nice but not as nice as Carp's and N8Dogg's HT) there is Noway I would consider getting rid of my SubM HP's for anything in the DIY field at the same cost. I know I cannot do it better than Mark or Jeff and the only way to get more would be to use less expensive products. I would kick my subs to the corner for a pair of OS's but I would not consider any DIY project for the same money. IMO if it takes 4 of anything to best one that is not better. I also think that a Cap S2 cannot be "bested" for the price if the same quality drivers and amp is used. Everything I just wrote is completely an opinion and is 100% subjective so please noone take it out of context.
I look at the total package value and to me for 3 grand the OS and Cap S2 is the best of the best while others can think that is crazy talk and I am cool with that.

For some a pair of HP's or OS's or S2's is more than enough and for others there is No such thing as enough and for those they will be chasing their tale till the day they die or get tired and give up which is where I am right now.
The only point I am trying to make is there is no need to put down one in order to compliment another. If this hobby really is about fun then All quality subs and sub projects should be appreciated for what they are.
Last night I had the house to myself for a few hours. I have been saving a BR of Korn Live at the Hollywood Palladium for some alone time to put my system to the test. It was the best experience I have had to date. Incredible, Amazing, and Badass all in one. It was recorded in DTS -HD and the venue had to be a small cement block building with concrete floors. The LFE signal light on my HP's amp stayed on the entire time that any and all music was playing. I have never heard or felt anything like this out of music except for something like the bass Meckanics. I listened at reference for the entire 16 song set. No listening fatigue at all but I was physically wore out afterward and my face hurts this morning from the constant perma-grin I had. The drivers have never pumped so hard on both the T12's and the HP's. For bass this concert is the first 10/10 I have experienced. I was almost at my bass limit for the first time in 41 years. This showing was superior to the Metallice Magnetic BR. So anyone that likes Korn this the BR concert to have.
Just my 2 cents mixed in with a little BS.
Chris