A:Answer I suggest that you do some research on the web, using recognized, expert audio review sources, then decide for yourself.. You should read:
1) https://www.homecinemachoice.com/content/pioneer-udp-lx500-ultra-hd-blu-ray-player-review compared to https://www.homecinemachoice.com/content/panasonic-dp-ub9000-ultra-hd-blu-ray-player-review ;
2) https://www.audiosciencereview.com/forum/index.php?threads/panasonic-dp-ub9000-uhd-player-review.14324/ ;
3) https://hometheaterhifi.com/reviews/video-player/blu-ray/panasonic-dp-ub9000-ultra-hd-4k-blu-ray-player-review-part-1-video/ ;
4) https://www.avforums.com/reviews/panasonic-dp-ub9000-review.15435#sectionAnchor57210 ; and,
5) https://www.soundandvision.com/content/pioneer-udp-lx500-ultra-hd-blu-ray-player-review
There are other reviews, but I found these helpful when I was going through what you're going through. In the end I bought the Panasonic because it got better reviews for the audio side of things. I have a large collection of CDs, and a fair number of FLAC music files. I wanted to get the player with the best DAC performance for for pure audio. I am extremely satisfied with the Panasonic's sound reproduction for audio media and audio files, despite its inability to play SACDs (of which I have several). And I've never seen or heard more spectacular Blu-Ray and 4K playback.