The magnetic cartridges used in turntables require a different type of input than the line input on your powered speakers. You may have seen these referred to as a phono preamp and its job is to compensate for the RIAA equalization curve that goes along with vinyl playback. Without changing your current set-up, look to Cambridge Audio, NAD, Music Hall or Project for an entry level phono preamp that would sit between your turntable and the powered speakers. It should set you back a $100 or so.
Regards,
Rich
Regards,
Rich