Hello
Thanks to everyone for your inputs.
Floor real state is not an issue.
I checked the BK webpage, and their subs, as you all mention, seem to be of great performance:value ratio in the UK. However, it seems like bringing them from UK to Finland is not so cheap, they ship only to UK and close by area for free. They didn't have a quote for Finland directly in their webpage, instead, they redirected me to a distributor (marek), that increases the price significantly, the monolith (which seems roughly comparable with the xtz 1x12) goes to €950, that's €270 of B&M commission which does not add any value, way too much commission in my opinion.
BK XXLS400 are €750, which is almost about the same as what the XTZ with the discount (those €700 per sub include shipping), and from those 2, I think the XTZs would be better option, because of what you suggested, flexibility of tunning either sealed or ported to adjust to the room, and also because I think is better a bit too much power than a bit too less, in case I move down the road to a bigger place and I find myself in need of that extra power.
I use at the moment an entry level Yamaha receiver, which only has one sub preout, but I am planning to get a miniDSP for bass management (will upgrade receiver at some point in the future).
So it seems like we have a winner, I'll order the XTZs and see how it goes
@tyhjaarpa , thanks for the input about the Amphions, I was already skeptical about them since their webpage has very little technical information, no frequency response graphs or anything.