Muffled Vocals/Sound Clarity

I recently received a pair of Polk T15 Speakers as a gift to use with an AudioTechnica turntable. I have the speakers and the turntable wired up to an Insignia bluetooth receiver. The sound is not quite right. I have adjusted the bass and treble. The issue is more noticeable with some vinyls than others. The main issue is that the vocals sound muffled, or suppressed. Like if a blanket or something had been tossed over a speaker. A friend and I have tried changing the wiring, changing inputs... The speakers are wired with speaker wire to the receiver. Any thoughts on what this could be? The speakers are new and about a month old. I tried using the bluetooth connection on the turntable to connect to a portable Bluetooth speaker, and the sound quality was much better. Any thoughts on what this could be?

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