I was in the same spot as you some months back. Decided to go the DIY route (with no prior experience). What I did was get Sanway's FP14k clone (as I saw notnyt are using them), miniDSP 4x10 HD (because it's a cheap alternative with a helpful community) and one LMS U driver.
After relentlessly updating tracking info for when the gear would arrive (live in Norway, so sea freight, haha), and sleepless nights thinking I might have taken water over my head I finally started the building process and made a very simple 3.7ft3 sealed test enclosure. The process was fun, and when I first turned it on and tried it, well, I was simply blown away. That night I was playing music insanely loud, drinking a ton of screwdrivers and probably had the happiest night of my life. As for the next day, not so much!
Luckily, the miniDSP 4x10 HD and FP14k clone will happily accommodate a second sub, so the second will be far cheaper than the first one as you already have the gear needed. After a weeks time, an order for a second driver was in. It'll soon arrive, and I'm researching my next two enclosures. I could have lived happily ever after with one in my simple enclosure, but since the second sub is so much cheaper than the first and the process of building so much fun, it was impossible to resist getting a second one and working towards something even better (even though I've got problems understanding how that would be possible). Also, there is this little thing called OCD and it really likes symmetrical looking systems,
As for the PB13Ultra, I heard it at a friends house with Klipsch RF7 and it's not even in the same league when it comes to sound quality. Yes, it's good, plays loud and all, but ... niet, not the same quality at all. The biggest problem with LMS is that the distortion is so low that YOU CAN'T GET ENOUGH!!! You just want to make your ears bleed and go deaf, but at least you'll be happy when it goes.
My brother has two Velodyne DD18 (not plus), and even though they sound great, I think that the LMS is even better.
My little sister's boyfriend is a huge dubstep fan, and when he heard what the LMS could do, he was basically showing a side to himself he has suppressed in my presence up until that moment when the bass kicked in; hands pumping, head banging, face grinning, hahahaha. It's just fantastic! The best invention since women I'd say!