If your KRC3 had configured the S3 as Proc, then you can use this input as a HT passthrough and connect it to MX135 FR/FL output,
the connection as below:
the "stereo audio out" on the oppo 205 connect to S1 on KRC3
the "5.1CH audio out on the oppo 205 connect to MX135
KRC3 output to FR/FL of KAV500
MX135 FR and FL output to S3 on KRC3, center and RR/RL channel output to KAV500
for stereo playback, select S1 input on KRC3, S3 for 5.1 playback.
the connection as below:
the "stereo audio out" on the oppo 205 connect to S1 on KRC3
the "5.1CH audio out on the oppo 205 connect to MX135
KRC3 output to FR/FL of KAV500
MX135 FR and FL output to S3 on KRC3, center and RR/RL channel output to KAV500
for stereo playback, select S1 input on KRC3, S3 for 5.1 playback.