A speaker the Devialet cannot drive...

[WSLam]

There is another solution. This would reduce the efficiency of your speaker though. Add resistance to your speaker So the amp sees a higher impedance. Mundorfs makes power resistors with 20 watt rating. Get 8 pieces each 4 ohms. Use 4 on each side and use parellel and serial connecting so you still get 4 ohms but rating goes up to 80. Open you speakers and solder them directly between bindung post and xo board.

I had the opposite problem before. My previous amp had a switching outputstage and was flat into 6 ohms but quite bright sounding into my duos 16 ohms. Could have used the digital tone control function but this added quite a bit of edginess to the high frequncies. Soldered a 10 ohm mundorf power resistor accross the binding posts of my amp to reduces the impedance the amp sees. Worked like a charme and sounded a lot better than using digital tone control

Interesting idea. 
Does this product do the same thing?
http://www.zeroimpedance.com/box.html

[hk6230]

Saw another similar product as above mentioned in May 2016 edition of Hi Fi Choice magazine: Spec Corporation RSP-501EX Real-Sound Processor.

[Duomike]

Guess they will also do the job just fine but it adds another box and cables. There is a transformer inside and i doubt it will be sonically as transparent as the mundorf supreme resistors. Your esl are very revealing and I assume and you can hear the slightest degration of the signal.