The gain/sensitivity specs are:
- Gain of + 12.5dB, for a total of 25.3dB (default) - Maximum power reached with a signal of ~ 2.2V RMS
Back to your question, there is actually no need to do much calculations as the answers are already in Amir's review:
Amir's standard sweep test shows 20 W/35 W, 8/4 ohm, two channel driven after 35s.
At 5 W, the left and right speakers (84 dB/2.83V/m), at your 2 m listening distance, will produce 85 dB, that's reference level, that means the LX505 will not go into power limiting if you listen to 85 dB average, that is very loud as you know.
The issue is the peaks, that will push the output level to far exceed the amps you listed to their rated output or even much higher. The question is, how long will such high peaks last, I have never come across peaks of higher than 100 dB from my mlp with all speakers firing.
Clearly the answer to your question is yes, but only if you push your LCR external amp pass 140 W 8 ohm, and stay at that level for 35 seconds or longer. It won't happen if the movies you watch don't have such prolonged peaks.
If the power limiting mode is not activated, then your surround channels can output between 0 to 140 W 8 ohms all day long. So either don't listen to contents that you will end up driving your amps to 140 W or higher for anywhere near 35 s, or get an external amp for the surround channels.