\The Radial is a rack mount line mixer and the PreSonus is a audio interface for a computer kind of an apples and oranges comparison. The Radial is basically a high end mixer and that's it. The PreSonus is a mainly an Analog to Digital convertor to send to your audio to your computer in digital format.
I haven't seen a statement as to whether the KL-8 does it's mixing analog or digital, but I'm going to guess it is an 8-channel 192kHz/24-bit audio interface with integrated digital mixer setup to be customized to this specific role. I believe the KL-8 can accept digital I/O on at least one channel and if it can send the full mix digitally to a computer, it can replace an audio interface in some applications.
Per pricepoint, it's actually a bit less than I'd have expected for a Radial product with this capability. Their deal is to sell quality and purpose built usefulness for gigging musicians, not to be the price leader. I'm personally not that into their designs, but I get why others are.