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The CD ouput is 75 ohm and the DI input is 50 kohm! no wonder there is such little signal.
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Actually, that's the way it should be. This is called a voltage bridge, where the sending device is acting as a voltage source and almost no current is being drawn.
In fact, a perfect voltage source would have an output impedance of zero ohms at all frequencies. This would result in all the output voltage from the source being dropped across its load, with no voltage being lost to the output impedance of the sending device.
Output impedance is basically the internal resistance of an amplifier seen at its output. The value can be slightly frequency dependant because of reactance caused by inductance and capacitance, but either way, a low output impedance feeding a high input impedance is desirable.
In its simplest terms you can view the internal resistance of a source (CD, in this case) and its load (the D.I box) as a voltage divider. The higher the output impedance of a CD, the more voltage will drop across 'it' instead of its load (the DI). This in effect means less voltage will be received at the load end.
If the input impedance of a load device is not significantly higher than the sources impedance, the signal will be reduced or 'loaded down' and its signal to noise ratio and frequency response will suffer. Certainly the load can become too much for the source to supply adequately.
Generally, a high output impedance requires close attention to cable lengths. The concern is that the high reactance (frequency dependant resistance caused by capacitance) of the longer cable, combined with a high output impedance of the source, creates a low pass filter which adversely affects bandwidth. This distortion of the higher frequencies increases with higher output impedance's.
The interconnects capacitance results in a reactance (frequency dependant resistance) that will roll of the higher frequencies. It becomes more and more a factor, the higher the output impedance becomes. This interface is actually the same as the connection between any source like a CD and the input impedance of a preamp, or a preamp and an amplifier, so the same rules apply there too.
Anyway, there's an old rule of thumb that says, the input vs output impedance of a voltage bridge interface should be a minimum of 10:1. That's minimum. It should be greater in a high end system to ensure high frequency preservation. So, if I fed a power amp with 10Kohm input impedance, I would just be OK with 1000 ohm output impedance.
Anyway.........., what Wayne said is true. Changing to a CD player for its balanced outputs is not likely to reveal any new detail and imaging.
BTW, the Arcam Alpha 9 has an output impedance of 50 ohms....
I happen to own one - beautiful player. Keep it.
brucek