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There's plenty of competition around, but the R400b justifies itself - not many subs under £1,000 utilise force cancelling benefits. It's a sealed sub, so will fare better than most ported ones, and build quality is excellent, which improves its ability to 'disappear' and not draw attention to itself. We've had them running in two channels systems on open days, and for the occasional demo.
If your budget allows then also consider JL Audio's E-110 subwoofer. The E-110's massively engineered/built drive unit backed up with high power amplification and strong cabinet helps reproduce a clean, deep, tight, fast punchy and dynamic bass. Fwiw, we've successfully integrated E-110 subs with similarly priced Aria 906 and curved SCM7's adding superb low end presence.
Well, if we're now doubling the budget, you can pick up two R400b subs! That's four 9" drivers. Depending how you're setting the sub up, two will give you better imaging. I know sub bass is generally considered to be mono, but in music, you can sometimes get bass in one speaker and not the other.
Never underestimate the positive effects and presence that two subs have over one... (budget dependent)