Subwoofers are a complicated purchase, and I have learned much over the years through trial and error. The most important lesson, as far as my experience suggests anyway, is this: there are good subs and there are cheap subs, and ne'er the twain shall meet.

That's because subs work really hard. They have cones that you can see punching out to generate soundwaves, and the soundwaves are so long that they have to punch far, hard and fast. This doesn't just necessitate tough cones: subs also need big, powerful motors in big, powerful magnets, lots of power (the amplifiers are usually built in) and great suspension systems. All of these things cost money, and if a manufacturer skimps on any of them the result is second class sound and abbreviated life cycles.

Because the wavelengths generated by subs are so long, the sound is non-directional, meaning you can't really tell where it's coming from. So a sub can be placed practically anywhere in the room that works aesthetically. I've had them operating effectively behind furniture and in corners, but some, especially down-firing models, frequently don't work on shelves or in sideboards because they vibrate so hard that the shelf or sideboard rattles.

These down-firing subs direct the sound to their base to be reflected back into the room, and need to be on a solid surface. Even some wooden floors can be a problem. Front-firing subs send their sound straight ahead.

Some so-called subwoofers have cones as small as 10 centimetres, but if you're serious consider nothing with a cone of less than 20 centimetres. This means that good subs are usually large lumps that are seldom tolerated by interior designers and their disciples.

Finally, subs do not make movie soundtracks necessarily more accurate, but by dialling up the bass they sure do make action flicks a lot more exciting. A lot more. Even old material you've seen many times, like Schwarzenegger in the Harrier in True Lies, suddenly becomes edge-of-your-chair stuff. But you'll need a level adjustment because some movies need more oomph than others, and some have so much bass you have to tone things down.

Lately I've been trying out the smallest $999 Kef Kube, with a single 20-centimetre driver. Kef suggests setting the level at six (of 10), and while this is perfect for the thundering cavalries in The Battle of Red Cliff and the car chases and shootouts in Ronin, it's way over the top for just about anything from the post office heist onward in Baby Driver.

The littlest Kube is a front-firing sub producing remarkable punch for its dimensions (29 x 29 x 31centimetres, 11.3 kilograms) and it's capable of filling even a large room with tonnes of bass. It works very hard. I had it beside the couch firing into the room ahead, despite which it regularly had the couch vibrating. By adjusting the level you can make it as subtle or as extreme as you want.

Its range is 34 to 140 hertz and you can nominate the frequency crossover or leave it to the sub's software. Bigger Kubes (with 25 and 30-centimetre cones) go down as far as 22 hertz and up as far as $1399. Kef says they're OK for mounting in cabinets.

All you need is a power outlet and an RCA cable (not supplied) to the amplifier. There's an equalisation switch allowing its placement in a corner, backed into a wall or freestanding. It's excellent value and looks about as good as a black box can.