The built-in speakers in the LP-R550USB are only 3.5W speakers so they are going to sound tinny.
The digitization of the LP by the system is unaffected by the size of it's built-in speakers.
Try playing the MP3's back on a different system with better speakers and you should hear much better sound quality.
Ok, so what you are referring to is the limit of the LP-R550USB device itself...
the only way around this would be to obtain a higher quality, professional unit to digitize your LP's.