1) When you send analog sound to a pre/pro, you cannot (well, you can, but it would defeat the purpose of sending analog audio in the first place) use ANY decoding or adjustment features in the pre-pro. You get 7.1 analog sound from the Panasonic player but you cannot get immersive sound if you send analog audio to the pre-pro. Plus: no room correction, no equalization, no LFE adjustments, no channel-by-channel volume or time delay adjustments... NONE of that. By sending analog audio to a pre/pro, you essentially turn the pre/pro into a volume control and source switcher... that's all (it's a little more complex than that, but not much). The reason for this is: if you send analog audio to the pre/pro, 100% of the changes you make in the pre/pro (channel balance, EQ, Audyssey or other room correction, phase or distance settings are made in DIGITAL... if you use those features, the pre/pro converts the incoming audio back to DIGITAL, makes the adjustments, then converts it back to analog before it goes to the speakers.
2) You won't hear any difference between the Panasonic disc player and the pre/pro for movie sound... the visuals of the movie are so overpowering, that unless there are bad sonic problems, movies won't sound much different no matter where the decoding is done. So DO NOT WATCH MOVIES WITH ANALOG SOUND... connect an HDMI cable from the main output of the Panasonic player to the pre/pro to get both audio and video to the Blu-ray input on the pre/pro and use that exclusively for movie sound. The HDMI cable doesn't have to be expensive, it won't affect image or sound quality... a cable of the right length from monoprice.com is all you need. The one thing that does matter is the high-speed capability of the cable... do not rely on "words" to tell you if an HDMI cable is fast enough... lots of them are called "high speed" from way back before 2010 when data transfer speeds were half or less than what they are today. Select the blu-ray input when you want to watch movies.
3) Music will have noticeable improvements from higher-quality digital decoding. BUT, you have to have pretty good analog audio interconnects running from the player to pre/pro. Analog audio interconnects make a lot of sound quality difference, but digital audio cables don't often have much impact on audio quality unless you replace a poorly designed cable with a properly designed cable...and properly designed HDMI cables don't have to be expensive. If you are a serious music listener (lights off, music on, and you often select music based on having the best recorded version of it, like the best conversion of a Karajan conducted Beethoven symphony from the 1960s on D.G. or getting the 50 year anniversary version of Sgt. Pepper's Lonely Hearts Club Band for no reason other than to hear the best mix that album has ever had. And you use music almost as guided meditation where you may not even notice pets or people entering or leaving the room even though you aren't actually sleeping... the meditative focus on the music can induce that state of mind. For THAT listening experience, use the best decoding and disable all processing in the pre/pro to keep the pre/pro from damaging the analog audio... that will also require engaging the pre/pro's Direct, Pure, or Pure Direct mode (different brands use different names for it). Those modes pass analog audio through the pre/pro without converting it back to digital... but be careful... in some brands, Direct mode plays stereo sources in stereo with LFE so the audio is digitized to create the LFE signal, so your audio is compromised. You want what ever mode your pre/pro offers that will not process the analog input signal.
4) Using the old optical (TosLink) or coax (RCA jack) digital outputs causes a loss of sound quality from high-res sources (newest movies and high-res music). This old digital interface that appeared in the 1990s is only reliable when you don't care about getting the best sound quality and you just want something that works.
5) You cannot get Atmos, DTS:X, or Auro-3D decoding if you do not send BITSTREAM audio from the disc player to the pre/pro. Another reason to use HDMI for movies or music video. BITSTREAM audio is only available via HDMI, coax, or optical... I already covered why optical and coax are not good in this era, so that means HDMI is your best option for moving movie sound to a pre/pro... by far.