The S6 quattro saloon is the quiet polymath of Audi's range. It lurks quietly in the listings, not drawing attention to itself but ready to do pretty well anything you ask of it.If the S6 were a person, it would be a man, old enough to be wise but still young enough to be athletic. He would be broadly educated, broad shouldered, with a good diet and a quiet air of unimposing self-confidence. He would as easily smile across the room at you as hold you with an expressionless gaze. His interest in art and ability to love would be limited, but his facility for life would be limitless.The latest S6 is lighter than the last model, the engine capacity has been reduced but the maximum power has been increased to 414bhp, and the bottom line is that there's very little to be grumpy about.The power is fed through a seven-speed S tronic gearbox which can be either fully automatic or shuffled with paddles or the stem. All the shifts are very quick thanks to the dual-clutch system which effectively has the next gear cued up before the current one is disengaged. The quattro four-wheel drive system normally has a 60% bias towards the rear, but can adapt almost instantly to put the drive to whichever axle is getting the best grip.On its standard adjustable air suspension, the S6 has a normal ride height 20mm lower than the standard A6. That automatically drops by a further 10mm when you're going over 75mph (which of course we never do in this country) or when the Dynamic or Efficiency modes are selected by the driver. Oh, and the suspension can be raised by 20mm at speeds up to 50mph if you need to get over obstacles or maybe navigate through a thin covering of snow.The other important techy thing to mention is the on-demand cylinder system which means that if the car is warmed up, in a high enough gear and not under much stress, it can close down four of its eight cylinders. It's all done in the name of efficiency while you're cruising along the motorway or A-roads. When taken alongside the start/stop facility it gives the S6 official fuel economy just shy of 30mpg. Not bad in a car that can clip through 62mph just 4.6 seconds after take-off.So you can see already that there's a lot of tech in there, but from inside and out, there's very little to make you aware of it. This truly is a stealth motorcar.Walking round it outside, you have to look very carefully to look for the signs that it's a performance car. The twin double-barrelled exhaust ports are a good place to start because the S6 badging is very subtle indeed.Other than that, it has the sleek profile of the A6, a couple of little air spoilers and a minimum of fussy styling. At the front there's the unmistakable look of today's Audis: a level stare and a gaping grille.You unlock the door with the keyless entry system. You settle into heated leather seats, adjust them electronically and stab the button to fire that big engine into burbling life.The spoked 19" alloy wheels paddle the road quietly as you move off and work your way through town, that clever suspension keeping you comfortable through the assorted humps, bumps and ridges in the street. And then the road and the world open up to you.Stamping down on the accelerator from a rolling start is rewarded with a shove back into the seat as the big car leaps at the horizon. Gears come and go with almost no sign, corners are absorbed, power is spat out. Everything happens without drama or fuss. Even the engine noise is kept to a background thrumming purr. It's a case of ultra-efficient point-and-shoot driving. Some may say it's unengaging. I say it's confidence-inspiring.The only thing I would say is that the standing start is a little hesitant. Of course if you want a quick getaway, you'll have disengaged the start/stop feature so that the engine keeps running, but there's a perceptible hesitation in the selection of gear one when the target gap comes in the traffic.The S6 is almost big enough to be a chauffeur's car. There is plenty of room in the main back seats and there's climate control for left and right. It's quiet, too. The noise from the engine is actually counteracted indoors by an active noise cancelling system that works through the audio speakers. Put simply, they flatten the intruding sound waves by hitting them with equal and opposite sound waves. A similar system works on the engine mounts too. Very clever.The boot is cavernous, at 530 litres, and there's the option of folding down the back seats to make it even bigger if necessary.The cost of all this technology and style is, as you might imagine, high. The list price is £54,505, but as ever with Audi, there are plenty of costly add-ons available and this test car I've been driving would set you back considerably more.The Estoril blue paint adds £905, the all-LED headlights with all the advantages of daylight colour, low maintenance and low energy consumption would cost you £1260. The technology pack which gives clever controls for things like satnav, audio and phone, as well as a string of collision avoidance features, would cost you £3250. The sunroof is nice, but it costs £950 and if you want the BOSE speakers you can add another grand.You might struggle to see who would buy the S6 quattro, but I think I can picture them now in my mind. It'll be someone who takes their time and does their research before parting with the cash. Their requirements for space, performance, image and price will be well-established to the minimum of tolerances. They'll have spent many hours going through magazines and brochures with a pencil and notebook, looking for the perfect match.The S6 Quattro is a carefully worked-out car for people who carefully work things out. Engine 3993cc, 8 cylinders Power 414bhp Transmission 7-speed semi-automatic Fuel/CO2 29.4mpg / 225g/km Acceleration 0-62mph: 4.6 seconds Top speed 155mph Price £54,505 Details correct at publication date
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