Usually settings and expectations, not hardware. Movies use every speaker in the room; music is stereo, so it lives or dies on your front pair and how you drive them.
The order I would change things in:
Play music in pure, direct or plain stereo mode, not a surround upmix. Upmixed music spreads a two-channel mix across five or seven speakers and smears the imaging that stereo depends on. Direct mode also bypasses much of the processing chain, which on many AVRs makes an audible difference on its own.
Keep the sub in, but check its level for music. A sub trim set for movie impact usually sits too hot for music, and the result is a thick bottom end that flatters explosions and buries bass lines. Some AVRs allow a separate sub level per listening mode; if yours does, set one for cinema and one for stereo.
Source quality matters more in stereo than in film. A properly mastered stream at a decent bitrate is audibly better than a low-bitrate one, and the difference is easier to hear on a bare stereo signal than under a film mix. Check the streaming app's quality setting; the default is rarely the best.
Speaker geometry. The listening triangle, toe-in and distance from the front wall matter far more for stereo imaging than for film, where the centre channel does the anchoring. Ten minutes with a tape measure and a small toe-in adjustment often does more than any menu.
Where does the rule stop mattering? When all four are done and music still sounds ordinary. Then the plain possibility is that your front pair was chosen for cinema duty: high sensitivity, dynamics first, tonal refinement second. Nothing wrong with that choice, it just shows on a solo piano. The targeted fix is a front-pair upgrade, or a dedicated stereo amplifier fed from the AVR's pre-outs so the fronts get better amplification for music while the AVR keeps running the surrounds. Not a new system.
Tell us the front pair, the AVR and what you listen to, and I will tell you which of the two upgrades matters, or whether the settings alone get you there.
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