Hey! Welcome to the forum. Nice gear you've picked up.
I believe this may be the case. AAC is not lossless. If possible, when importing CD's use ALAC. Same when buying digital music online (get ALAC or FLAC formats, if not WAV).
Spending on a USB cable will be a 100% waste. Might as well put that money in the fireplace. Speaker placement would be my first stop, but, this seems to be a non starter. That leaves, EQ. So, we have the winner
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The thing is, without a baseline understanding of the system performance, we're just shooting in the dark. Do you have any room response measurement capability?