The BK Gemini is a sealed box, 10" driver, 150W amplifier. It's a good sub, but be aware of it's limitations. After all it is only a sealed box, 10" driver, and 150W amplifier. It's superior to the Q Acoustics sub though. I'd be looking at the XL200 ideally if it was me I'd get the best BK sealed box which is the XLS400. But it's always better to have a subwoofer that is too good, then having one that you are pushing too hard and it sounds bad. Like a motorcar, a old 1 liter car from the 80's could just about handle motorway driving speeds, but you really are pushing it, thrashing the engine, it's shaking all over. But 1.6injection handles it so much easier, at 70mph the engine is at a nice cruising speed, it's better build so not shaking.
The BK Gemini sub will be fine in a smaller room, for TV use, and for music. But if you want to feel deep clear bass at higher levels, and not worry about bottoming out the driver, running out of headroom, getting higher levels of distortion then you'll need something better.
I have two subs (two systems) One is a 12" driver, sealed box, 400W, Another is 13.5" driver, sealed box, 1300W. Personally I prefer sealed box subs now for smaller rooms (having ported before) But that's another matter. If I had a larger room and this is for movies, then I'd choose the 13.5" driver, ported box, 1300W version of mine (which I had previously, which I sold to buy the sealed box version) That was a movie monster, you could feel the floorboards flex and it made you feel funny as it could do output 10hz material.
As for white speakers, I would only recommend white speakers for ceiling/atmos/side/rears. Not front/center/sub. White sticks out, and with the lights dimmed, the white speakers will be very distracting. Had a person whose wife wanted white Monitor Audio speakers, I said no get a dark colour, he obeyed his wife (lol) when he got them both himself and his wife said they are distracting lol. So what did they do? Move the white speakers from the front to the rears, then buy the same speakers in darker colour for the fronts. haha.
As for bipoles, I don't think I mentioned any specific models. Generally bipole (and dipole and tripole) are generally more expensive than the treble/driver speaker counterpart because these dedicated speakers have several more drivers. From double to perhaps triple. I use bipoles for sides and rears, each speaker has two treble drivers, and two mid/bass drivers. I think they're £600 per pair new (so £1200 for four) but I paid something like £400 for all four (either second hand or ex-demo) I have another set of speakers, dipoles, which have two treble, two mid, and one bass driver.
these are my main speakers are these. Buy second hand, and get better quality speakers.
http://www.avsforum.com/forum/attachment.php?attachmentid=662250&d=1428833584
Originally there were £3500 a pair. I paid £725 for them.
Q Acoustics are pretty small speakers overall. I wouldn't recommend going smaller than them, and get the largest Q acoustics standmount you can. Don't go for ones where they home theatre in a box size (small speakers)
If you want every single speaker identical, according to people like Dolby/DTS which they recommend. So if you wanted M&K speakers, you'd have 7 M&K S-150's. However due to the shape, or personal preference having 7 of those is not possible. For me, a huge square center won't fit, so I have to choose the horizontal ones (Celestion A4C) rather than another A3. For side/rears they are too big, and I prefer non direct firing speakers, so I'd choose bipole or tripoles (M&K do have tripoles)
Also having 7 of my main speakers all round is not possible for obvious reasons, unless I had a conference room.
I'll be putting a ad in members market..