Advise on whether to upgrade to 440, Astra or something entirely different

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I'd say @Jean-Marie is right on the money here: it's likely to do with how the perceived tonal balance of our hearing changes with volume (https://en.wikipedia.org/wiki/Equal-loudness_contour), rather than a "sound quality" problem related to the equipment.

The question is: do you like how the system sounds when you turn the volume up? If so, I'd definitely experiment with a bit of bass boost at low volumes before thinking about spending money on new equipment.