Very impressive!

how do they sound?

This is a very interesting project.

But what kind of amplifier do you use? Do you use the original B&O soundprofil?

They sound extremely good, I will try to make a comparison with the original and come back. With the Sonos trueplay calibration, the frequency range will be flat.

The kit include an Ikea Symfonisk amplifier and electronics (which is made by Sonos). This consist of a very powerful DSP which will calibrate the Beolab 4000 speakers to a flat frequency spectrum and a multiroom wifi active two way speaker.

So it's basically an 130€ Ikea Symphonisk shelf speaker guts connected to the Beolab 4000 drivers?

I think I've seen the same mod with a Beovox C100 somewhere.

Just to be clear, no Beolab 4000 drivers are present. The Beolab 4000 speaker elements are direct connected to the Sonos amplifier and all is 100% integrated in the Beolab 4000 cabinet.

Really?  I thought it just listened to the output and calculated a boost as a percentage of the input, i.e. you could be driving cardboard (or flax!) with tubes, for all the DSP algorithm cared.  This is how Dirac Live can work inside a MiniDSP DDRC-24 (e.g.): Dirac knows nothing about the speakers or the amp, only what it hears from its microphone input when it sends out a known signal sweep.

And "speaker elements" == "drivers" colloquially so yes, s/he is indeed hooking up the inexpensive Symfonisk amp to the 4000's drivers and bypassing or removing the electronics.  BeoWorld description claims 35 watt x 2 (ICEpower version mk.2), and supposedly the amplifier on the Symfonisk speaker is the TI TPA3116D2, a two channels x 30 watts per -- so it could work OK unless you crank the volume up, maybe? [Edit: the DSP will take some headroom too.]

P.S. @carljh, kindly use your first sale to buy a gold membership here, in order to post to sell stuff.

You calibrate the rebuild speaker with Sonos Trueplay which is included in the Sonos app for Iphone and Android. The speaker play a frequency sweep which the phone pick up and adjust the speaker for a flat frequency response.