That series of Kef was hard on amps that did not have high current delivery or had sensitive protection circuits. The 4 Ohm load draws a fair amount of current and the KUBE boosts the bass output so the current demand at low frequencies is considerable. This often caused amps to cut out. That being said we used to demo them with fairly modest amps such as the 25 watt Mission Cyrus One to show how efficient they were (89db). You need something of very high quality as they are revealing speakers but beyond that as long as an amp is well designed and happy with 4 Ohm loads it will do the job. More power will bring more impressive results. I remember the big 300-500 wpc Perreaux amps (same distributor as Kef in Canada at the time) did a great job. The best I ever heard them was on the ML 23/23.5 but that could be said for a lot of speakers. I currently use a Yamaha RX-V1 AVR on my set in the living room and it works well, not surprising given it weighs about 62 pounds and is made to drive 6 speakers at once.