Are you sure you have the tracking force set correctly? Get a digital scale to check.

Don't expect vinyl to sound as good as digital, it's just different with it's own quirks and records can vary in mastering and pressing quality.

You could try switching from the stock conical tip stylus to an elliptical tip stylus.

Upgrade the speakers first before considering a cartridge change since the speakers can be more of a sound quality bottleneck than the AT91R cartridge could be. A phono preamp upgrade may help a bit for the entry level Edifier speakers however it will make more of a difference after a speaker upgrade. Art DJ Pre II or iFi Zen Air Phono.