In fact, the two have significant differences, especially in the bass. Yes, both share the same CLS Generation 2 electrostatic treble/midrange panel, housed in ML's rigid 'AirFrame' curved aluminium housing, and both have a broadly similar footprint, but where the Source features a single 200mm bass driver, the Purity sports a pair of 165mm units, driven by a built-in 200-watt amplifier.

There's a three-position bass control at the rear of both speakers, for +3dB, flat and -3dB, and its best to experiment. There's even a set of speaker terminals for those who want to drive the stators from an amplifier and that turns the internal amp into a subwoofer driver, although that's effectively irrelevant in this case.

Space to breathe

As ever with MartinLogan speakers, the Purity needs a lot of air - a good metre from the rear wall and half a metre from the sides.

As a high-end partnership, the two fit snugly together, although there's one small sacrifice in reality - the MartinLogan speakers don't accept a balanced input, so although the Krell KID takes advantage of the differential output of the iPod, that advantage is not passed on here.

Still, unless you have to use more than 5m runs of interconnect cable 'twixt KID and Purity, it's doubtful you would gain much by going balanced anyway.

The limitations of compression

Part of the issue with any dock is the quality of the recordings made to your iPod.

These can be so variable that they can make or break a sound, and many's the time low-fi systems actually do better than their high-end brethren in this context because small, cheap speakers are less demanding and thus, do not highlight the parts where data has been sacrificed at the altar of disk space.

In contrast, a full-range audiophile-grade system can throw the limitations of data compression into sharp focus.

Choose your formats wisely

So, if you are planning to use a take-no-prisoners replay system, you need to be just as resolute in your choice of format; AIFF or WAV give you bit-for-bit transfers, but with the concomitant disk-eating properties of storing 650MB or more per CD, Apple Lossless (as the name suggests) works like a zip compressor and does not interpolate the music itself.

You are still looking, however, at nigh on 300MB per CD.

Then, there's AAC and MP3; consider 160kbps AAC and 192kbps MP3 files as a bare minimum for use with the ElectroKID and if you can go higher (both go up to 320 kilobits per second), do so. You still get hundreds of hours on a single iPod (400 hours of music on a 60GB model sounds about right), but at least they are all listenable.

There's even the choice of iPod to consider. Naturally, your iPod has to be second generation or later to sport a docking port. But there are also some who reject the iPhone and iPod Touch on audio grounds.

Exciting sound

There's a symbiosis between the two non-Apple products that bespeaks quality. In other words, the KID and the Purity sing together beautifully. The Krell KID is more of a preamp than a dock and the improvement that it brings to the Apple sound is quite remarkable.

You get a vivid, exciting and clean sound with a surprisingly fine soundstage... something far better than the flat, pinched 2-D sonic disappointment that comes when you usually hook your iPod to the hi-fi. All this without even tangling with digital code must mean Krell is doing something right.

The sound is perhaps drier than typical Krell products, but is still an excellent presentation.

The MartinLogan side of things is not too dissimilar from the performance heard in the Source loudspeaker we recently tested.

The extended, smooth treble, the open mindrange, that effortless sense of disappearing loudspeakers and the well-integrated dynamic bass are all re-issued here.