Originally Posted by ROBSCIX
..then a logitech Z-5500 headphone jack?
Those speakers are not considered that good for sound or electronics. Don't get me wrong they can be great for some for the power/price factor.
You can be assured there are much better sounding systems out there.
This is a bit biased in nature. The sound doesn't run through the Logitech's hardware in any way. Just the cable from the computer, which meets up at the Z-5500 control pod.
@OP, there definitely is better, but you'd have to spend some cash. Along with this, you have a REALLY easy to drive/amp IEM, and you would hit diminishing returns in amplification really quickly. I'd almost suggest sticking with that setup, unless you want the very best, which would be expensive.
If you want better anyway, you can look into the RSA Protector + balanced recabling. You mentioned you have portable sources as well as the computer, and the JH13+TWag has been paired well with the Protector. (Source) This, plus the line-out from your X-Fi, would make your setup sound amazing for years to come.