You need a phono preamp between your TT and your all in one Marshall speaker system. Because a phono stage is the one thing your Marshall doesn't have. Phono preamps start at around 50 bucks up to 5 figures.
Connect the two RCA leads and ground wire from your TT into the phono preamp inputs and ground terminal. Then run a pair of RCA leads from the preamp into the analog inputs of your Marshall. That simple.
Connect the two RCA leads and ground wire from your TT into the phono preamp inputs and ground terminal. Then run a pair of RCA leads from the preamp into the analog inputs of your Marshall. That simple.