Despite the Sonos using Class D amplification, I'd still try and choose a speaker that presents it with a nice easy load and make the most of the power it has. The entry level range from most manufacturers are designed to be an easy load for most amps - Monitor Audio's Bronze range, Dali's Zensor range, KEF's Q Series, and usually B&W's 600 series are ok too. Don't just spend more because you can.
If you really want a wider choice of speakers and not want the worry about suitable amplification, try the Cyrus One amplifier, which will drive most speakers under £2,000 quite happily. In fact, it drives Eclipse TD508 MkIII speakers well too, and they're 82dB sensitivity. Whilst it won't be quite as small as the Sonos, it will still be smaller than most amps, but still pack a real punch.