Run direct from ML 390s CD player without preamp?

I did this with my ML-39 and ARC VT-100 with good results.
There are those who claim using a preamp results in
better sound. Try both and decide what is right for you.

Advantage is less circuitry between you & the recorded signal. Disadvantage is the impedance (and load input sensitivity) is not always matched between the source and load. The output impedance of the 390S is low, ~10 ohm / maxi output ~4 volts. Check out the input impedance of the CJ (usually quoted in the specs).

Or, as Dweller suggests, try it. Much simpler. If you hear bass notes at lower volumes, the 390 is driving the amp. If not and the sound is anaemic, then it's not driving it & you need a pre.

Also, you would need a pretty good preamp not to reduce the sonics of the ML CDP, which has a pretty good volume control in it stock. You would need to audition a few preamps to see if they better the sonics versus going direct. I am about to audition a few of the CDPs that allow you to go direct and see what they can do, but I also have four different preamps to see if any can add to the sonics.

Happy Listening.