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.
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.