The Monoprice Monolith sub does represent good value. They've got all the useful stuff such as variable phase and a healthy power output. It's only available in black ash vinyl wrap, but that's because they focussed on delivering performance over aesthetics for a retail channel sub. Aesthetic considerations first is what gets us products such as Bose Acoustimass speakers.
Compared to the Dali and Monitor Audio, aside from the 10" driver versus 8" on the other two, the other big size difference is the box itself. It's ported (bass reflex) which requires a larger volume of space inside the box to work. That means a bigger footprint and a taller box too. Being a ported does mean though that the sub is louder per Watt than a sealed unit of the same driver size. That comes in really handy when there's a pretty large room to pressurise.
One thing to bear in mind with porting is how it interacts with room boundaries.
All speakers get louder the closer they're put to a wall or floor. At higher frequencies (above around 200Hz which just above the higher-end of the male vocal speaking range) the effect of the lift is +3dB per wall/floor/ceiling. By 3kHz the effect has tailed off and other factors such as first reflections become more dominant. That's why room corners are bad for speakers. They change the tonal characteristics. At lower than 200Hz the effect is larger. You get up to +6dB per surface. The shift is gradual, but by 50Hz you're looking at +6dB per. This means that if you put a ported sub in a room corner then you'll get 6+6+6 dB of bass lift. That can change a sub from sounding tight and tuneful into a bloated mess at higher volumes when the action kicks in.
Fortunately, the folk at Monoprice designed their subs with this in mind, and so it ships with a foam bung to block the bass port. If you read any reviews or watch any videos where they have measurements which show the results of the sub with an sealed port as well as open and you notice the line tailing off for sealed, don't panic. The balance will be restored once the sub is placed in a corner. Subs that are sealed by design are intended for placement in a room corner, or at least against a wall.
Whether the V2-10 is right for you is a decision only you can make. It's large and black. Pretty powerful (but you can always turn it down) nd just shy of £600.
If you're looking for an alternative closer to your budget then I'd suggest having a look at the
BK P12-300. This is a sealed 12" sub with a 300W amp. The version I'd go for is the FF which stands for Front Firing. Like the V2-10 and the Dali and MM subs, the driver faces into the room rather than at the floor (DF - down firing). There are pros and cons of each, but in your house the FF is going to put a little less vibration into the floor and so the neighbours might be happier. The price is £435 with a choice of finishes including satin black, light oak and silver.
BK started out as the manufacturing facility for the designs of a number of well known brands including REL. The company now sells its own products direct to end users, and it has a bit of a following. Since it sells direct there's no dealers margin to add on top. This makes these subs very competitive. The P12 would be closer to £700 if sold via AV stores.
I'll cut to the chase and tell you straight that a P12 isn't going to go as loud as the Monolith. In a room bigger than yours this might be a problem, but I doubt you'll have an issue unless you like your bass overblown and boomy. I'll give you an example of that. I have a customer with a large dedicated cinema room. We're talking about 30ft x 16ft. For a long time he was running a Paradigm PS-1000 sub (275W RMS, 10" driver, ported) and he likes the sub set way too hot for me. The bass drones and booms, but he loves it like that. Well, about 12 months ago the Paradigm died. The issue couldn't be fixed. A temperature sensitive component inside the transformer shorted out and there was no way to get to it. My repair guys are good, but even we have to throw in the towel now and again.
Ticked off, he went in to Richers and they tucked him up with an SVS SB-1000Pro (375W, 12", sealed, £580). Given his room size and the way he likes bass it's totally unsuitable. The sub is lost in that space. I found him a replacement Paradigm and happiness was restored. I can hear what the SVS sub is doing. It's tight and deep when set correctly. Nothing like the boom and overhang of the Paradigm. In my room, the SVS would work just fine (16' x 11' 6") But for my customer, in that large room, and the way he likes his sound, it just doesn't meet his expectation of bass.
If you can splurge, and there's a chance that you might move to a place with a much larger room, then the Monolith would be worth the stretch. However, if you're trying to keep the budget around the £400 mark, and one of the finish choices meets the required WAF, then you might want to take a closer look at the P12-300.