klaudio,
the KAV 400xi is not a fully balanced design,so why would you want XLR ? If you use the XLR,in a non-balanced design unit ,the preamp section will get 'saturated'.So, unless both CDP and the AMp are trult balanced design ,it would be better sonically to use RCA, instead of balanced out[balanced out tend to have higher output,a lot of nonbalanced preamp/amp sound its best when the output from the CDP is less than 3mV]so this should be your primary consideration,or better still look for one with variable output,like METRONOME, WADIA