Monolith is legendary movie sub which still today shouldn`t be forgotten!
Measured performance for the 300w version was really good for the price. -3db @ 19/20hz (FF/DF) same as with 12.17 . 20-25hz performance is very close between XTZ 12.17 (maximum output) but then the XTZ will have clearer edge over Mono at 31,5-80hz. I think the new class d-amp and dsp chip can be tuned more freely plus the driver is better than in BK and cabinet design has been improved aswell. So newer design on XTZ has some advantages, plus the tweakability and more wife friendly size. It still doesn`t make Monolith bad choice (grade b pricing or 2nd hand!).
We got to remember most of action movies don`t have things going at 20hz or under all the time. It`s mostly higher +30hz. Some movies more and some less. So while it`s still important that sub performs great at the ~ 20hz, it`s important it does it also at higher. Feel free to have different opinion. I have seen many posts about SVS (PB1000/2000) cause they tune their subs to get maximum SPL around 16-20hz and maybe because of that they miss some mid/upper-bass punch.
Size matters with subwoofers and that is partly the reason why Mono performs so great, it`s larger than XTZ and many wifes won`t allow the high DF model in house. Mono FF is closer to XTZ, but depth wise still noticeably larger. Monolith weights 50kg, as XTZ 25kg.
There is BK Mono Plus model with better Lab12 driver and beefier 500w/1000w amp. Sadly no measurements of this but owners here love it and i`m sure it`s even better than the old classic.
Here is the measurements for DF (there is FF too somewhere ), notice that this amp has changed if you now buy the 300w version. It has some protection that the driver won`t bottom etc. See post #4 for explanation of results;
Bk Monolith-DF - Home Theater Forum and Systems - HomeTheaterShack.com
If someone is intrested about in Monolith DF there is two 300w versions and one 500w version listed for
406£ and
494£ plus delivery around 30£ or so.. Grade b`s but new and full warranty from BK.
Monolith Plus -DF Subwoofer in Black | eBay
Monolith-DF Subwoofer in Black | eBay
Monolith-DF Subwoofer in Black Ash | eBay