How to hook up an AV amp with Logitech Z-2300 2.1 powered speakers

Don't use the sub output.

You'll need RCA male to female mini jack to plug your Z-2300 to analog audio output from your receiver and not the sub.

I don't know what is typical voltage of X-fi, the impedance or the amount of current available to drive the load of that unit, but since the power of the various signals does not necessarily apply with the output voltage of the equipment and the Z-2300 is design for to handle 1V which is standard nominal voltage, therefore it's safe to assumed like a preamp, and that is 1Vrms.

As I suspected, you say the Amp put out 4 volts to sub? Then it's too high, you'll need an attenuator for it to work, plus the frequency difference from line signal to LFE. Tuning the sub may require a little more attention than one coming from line level output although the compensation might be more rewarding as well.