There can be only the following reasons for mudddled sound. BTW what do you exactly mean by muddled sound?
1. Speakers out of phase. Is the speaker connected correctly. The cable connected to the red binding post on amp connects to the red binding post on the speaker. Same goes for the cable connected to the black coloured binding posts. This is minimum required for both the speakers
2. Amp connection for left and right is out of phase (very unlikely). I have seen this issue with Fosi V3 Mono amplifier, but here if both left and right channels are reversed, it doesn't matter.
3. Speaker connection inside for one of the speaker is incorrect (phase reversed). This is also very very unlikely
4. Your source output has either the left or right phase reversed (very unlikely).
5. One of the RCA cable connecting the source to the amp has wire reversed when soldered to the RCA connector. Somewhat unlikely, but I have seen this mistake in one of my commercial RCA cable. This can be tested easily. Take the two rca cables and with a multimeter make sure the center pin of one end shows continuity with the center pin of the other end. Similary the outer shield at one end should show continuity with the outer shield at the other end
6. Poor room acoustic. Try having the speaker placed on one of the longer walls
7. Speaker placement issue
Out of these speaker, placement is the most frustrating, and room acoustics can make this almost impossible without some treatment. Rear ported speaker like Linton makes it difficult in a small room as you can't go very close to the wall. Treatment of the room can be as easy as filling the room with furniture, carpet, mobile diffusor panels. With small rooms, especially without furnitures, it can be a bit difficult without measurement and without treating the first reflection points. Points 1 to 7 can be tested by playing samples from
https://www.audiocheck.net/. BTW it took me few years to get my placement to a satisfactory level. I too had complications like asymmetric room, large tv with the panel made of real glass. Full height glass windows, etc.
You can also play a track that has the singer voice at dead center. If you play the track and find the vocal at the center than you can rule out phase issue. Phase issue is the single biggest reason for confusing muddled sound. However I don't know what you mean by muddled sound.
NOTE: I think the song "Little Room" by Norah Jones has the vocal at dead center.