Page 51 of the manual:
Note for speaker detection and size setting
When you set the front speakers ("L", "R") as "LARGE"
- The subwoofer also makes audio output in the bass range when you perform stereo playback of analogue and PCM sources.
- The subwoofer sends out only the LFE (low frequency effect channel) signal contained in 2-channel Dolby Digital or DTS sources when you play them in stereo.
It appears that all Panasonic receivers have the same design flaw. I verified the same problem with SA-XR55 (optical or coaxial connection) and SA-XR700(HDMI connection). Essentially, if you have a 5.1 system with front speakers set as Large, Panny receivers are unable to play 2-channel sound (e.g., stereo PCM such as compact disc) properly. You will notice that sub is active even for 2.0 content that contains no LFE information, e.g., compact disc. The only option is to manually turn off the sub whenever you listen to stereo music on your HT system.
Sub cannot be active when you play stereo music over Large speakers. By definition, sub signal should be zero. It is not clear what kind of signal Panny sends to the sub in this case; however, it cannot serve any purpose other than to muddy the sound. What is "bass range" that Panny refers to in the manual? Front speakers should be in charge of reproducing the entire frequency range, and no other speaker should be active in this situation.
It is remarkable that Panny engineers could be this clueless about 2-channel playback. Clearly, not every customer will have front speakers set as Small and many customers will want to use their HT system for both movies and music. Those with front speakers set as Large will have to manually turn off sub whenever they listen to music. Totally ridiculous.