Since I switched to TIDAL, someone asked whether I felt it sounded flatter and quieter than Qobuz. Initially, I did have that impression and wondered why.
Later, I realized that TIDAL has a Normalize Volume option enabled by default. If you're comparing TIDAL with Qobuz, you should turn it off (Settings → Audio & Playback → Normalize Volume → Off).
For some reason, TIDAL does sound flatter with Normalize Volume enabled, even when the playback volume is matched. After doing some research, I found that TIDAL uses its own loudness normalization system rather than standard ReplayGain tags. ReplayGain simply adjusts playback gain and does not alter the audio signal itself, whereas TIDAL's normalization appears to involve additional processing that many listeners perceive as reducing dynamics. To get the best possible sound quality from TIDAL, I recommend turning Normalize Volume off.
I also found that TIDAL Connect (TC) sounds better than playing TIDAL through the Volumio app. Interestingly, I found the opposite to be true with Qobuz: playing Qobuz through the Volumio app sounds better than using Qobuz Connect (QC).