I have a BK XXLS400-DF...
It's a great sub. Good balance for music vs movies. Yes, if you're a real bass hound for movies the Monolith might be better, but it's also huge - the clue's in the name
I've found it very responsive and not at all flabby and very easy to set up and pretty forgiving of placement. The price/performance is excellent - yes there are better subs, but they are a lot more expensive, you'd probably end up spending around £250 more to match it. It's solid and well built, I've got an Oak veneer one and the finish is excellent - it almost looks like a piece of furniture, which helped sell it to my OH. It's got low level and high level inputs, which makes connecting it very flexible, and the A/B amp really drives the 10" speaker hard and fast.
BK Elec are very helpful, I wasn't sure which size sub to go with and they gave very honest advice, didn't try to upsell me to the bigger and more expensive monolith, just stated which of their subs were suited to my room size and the pros and cons of each, and what I should consider when choosing.
I have no complaints at all about it, or the company and would heartily recommend it.