I have previous KS-3 Knock sense model. It use same chip as VEMS. KS works only from frequency. It do not knew nothing about rpms, load etc. My ears, when Peltor headphones used, is more reliable.
Also I have sound amplifier from Phormula. I must say my self-made amplifier with filter is much more handy. Amplifier from Phormula has very primitive construction - it is wide band: schoolboy electronics. So my hearing is loaded with every kind of engine noise.
Also Phormula knock listener has clear sound until you attach KS to them. KS is powered from cigarette lighter and then alternator and other noises comes in.