you can't bi-wire thiel speakers it simply does not even have that option, whether it will make the sound muddy is secondary.
Bi-wire speaker cable can be used in speaker which does not support bi-wiring if the cables have spade connectors, as for the KW250s i think is a brilliant amplifier but one needs to move up above the thiel 1.6 to really understand how it sounds.Audition it with the CS 3.7 and it will start to perform.
I use nordost cables and i really like them, they are the most transparent cable i've used in my system.