Thanks for the help and suggestions; all input is appreciated. Below is the rough layout of the room. The center, fronts, and surrounds are all in-ceilings and the rears are bookshelf speakers hidden above my upper kitchen cabinets.
I only have a couple options for sub placement - the only two possible locations inside the room are where I labeled SUB 1 and SUB 2. Both locations are constrained because of some windows that I can't block. The sub boxes would need to be 23" H x 25" W x 32" L at most not to block the windows and fit next to the fireplace, but that would give me something in the range of 6-8cu ft each. However, I'm a little concerned about doing the in-rooms because my woodworking skills are not very refined, and don't want to end up with something that looks terrible or end up working on this for a year. I put together an estimated bill of materials for two ported subs based on the UM15 and came up with total of approximately $1100 using an iNuke 3000, which doesn't include the cost of router or any other tools.
As an alternative, I could do a IB sub in the attic right above the MLP on the sofa, but if I setup the simulation correctly it looks like I'd need four UM15s to get the same output around 15-30Hz as the two in room ported subs. That would presumably simplify the cost/complexity of the boxes but require that I buy a iNuke 6000 and four UM15's at a rough estimated price of $1200. As an alternative not shown in the drawing above, I suppose that I could also do two IB boxes above the couch adjacent to the two surround speakers. I don't live in the country and do have neighbors, and the main issue that I understand reading about an IB setup is noise outside of the house because of the placement in our attic.
Below is my attempt to compare two different ported subs (6-8cuft per sub) and IB subs (2 and 4 UM15s):
For comparison, I am still considering buying 1-2 PB2000's or a XV15se, because I'm not saving a huge amount of cash going DIY and would have something turnkey within a week or two.
Any additional suggestions or considerations?