Krell KRC3 preamp

My suggestion:

the "stereo audio out" on the oppo 205 connect to KRC3

the "5.1CH audio out on the oppo 205 connect to MX135

use a Goldpoint SW4 Main out connect to KAV500 FR and FL input
http://www.goldpt.com/sw4.html

KRC3 output to Goldpoint SW4 Line 1

MX135 FR and FL output to Goldpoint SW4 Line 2, center and RR/RL channel to KAV500

for stereo playback, power off MX135, switch Goldpoint to Line 1 to use the KRC3 as preamp

Goldpoint switch to Line 2 for MX135

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.

@ei001h
You need to remove the top cover of the Krc3, look for a red dip switches labeled "Input configure" and set the S3 to Proc. position.

My suggestion of using a Goldpoint SW4 is easier to A/B comparing two preamps. 
If you decided the KRC3 is better for stereo playback, then set the S3 to HT passthrough will be the best way for connection. you can use the KRC remote control to switch input, most importantly, the connection is much simpler with one less component in the signal path.

can you please answer the second part of my question about additional RCA connections from MX135 to my amp?

Sorry I don't understand your question, can you more specific?

If you want stereo playback through MX135, just select S3 input on KRC3;
for stereo playback through KRC3. then select S1 input.