Did you check the cables too? So no matter which half of the cable you use, we can say the left (black) output from the preamp is bad?
so we can say the red & black cables between the preamp & speakers both work, as long as you are connected to the red preamp output?
Now, we still don't know if the preamp is getting a bad signal or if the preamp itself is bad. So switch the inputs to the preamp and if the buzz moves to the other channel we know the problem is before the preamp.
And if you can get a good signal through either preamp channel, you can swap the connections at the turntable-end Assuming the cables to the turntable can be disconnected at both ends, you can swap the connections at the turntable-end to determine if the problem is in the turntable or the cables.
...Since you have one complete-working path from the turntable to the speaker you can swap things around and narrow-down the problem. Just try to keep clear in your mind what signals & paths are known-good and try to keep track of what you're doing, and if you loose track repeat the experiment.
Hopefully it's a bad cable.
A missing ground between the cartridge and preamp (or a cable with a broken ground) could cause a loud hum/buzz. Or if you installed the phono cartridge, one of the connections may have come loose.