Lets talk about why bigger is better, You remember when I mentioned my HT build with 20 18" subwoofers and 28kW rms of power runnning them? Sure part of it is building what is easily in the top ten most powerful subwoofer systems in the US which could literally cause structural damage to my 3500 cuft room and would be the equal of over 40 PSA 1801s there is also a legitimate audio reason for this. At 16hz this system will be loafing along at 130dB not stressing anything. The harmonic distortion will be very low and the remaining headroom would be very high and the chances of port noise at any volume level a human could endure are almost nil.

Your room is big and open so there will be times that most any commercial sub will be asked to produce its maximum effort in some action movies etc. It is like asking the F150 to tow a little over its capacity on occasion, it is much nicer to have a F350 that just laughs off the load and doesn't even stress its Truck Nutz. The F150 will be making ugly sounds letting you know it is unhappy but the F350 will just be purring along asking "is this all you got?". In that big room there are going to be times you want/need a little more and having as much as you are willing to buy will be welcome at those times. The key is how often are you gonna pull that big trailer and if the F350 is worth it to you for those times.

No matter how capable a subwoofer is it should be calibrated within the full system and your Onkyo will do it with its software (I forget the name of their proprietary room correction software they use after they dropped Audyssey). So even if you had 10 1801s they should never be obnoxious, they would just loaf along and not be stressed. (I will say most AV nuts run their subs "hot" by that I mean add 2-3db or more to the sub track in the AVR settings after it is calibrated) More is never an issue because the throttle never gets pushed as hard.

Coming from no sub means any of the ones you are looking at will like seem a huge upgrade, if you have the money go big the first time and I am pretty sure you will never regret it except when you move it.

Outside of the return shipping with SVS I prefer the 1801 to the PB12Plus just because it has more raw capability in the mid-bass area something very important to the mental impact of a sub. The one thing there is no denying is the SVS looks like a more commercial/finished product and that is one of the things you are paying extra for and why the PSA has more performance potential. Better paint job/weaker engine. This is something you have to make a personal choice about.

From my POV you have to ask yourself how often do I watch a movie at (or above) movie theater sound levels. The rarer that is the easier it is to go with one of the cheaper subs. Between SVS and PSA you have to decide how important looks are to you and your SO as well as how hidden it will be.