I think you're def on the trail of a good sound, Ratvn.

One thing to remember about "good sound" though, is that not everyone likes the same thing.  So go to please yourself first, and then some listeners, and you should be good to go.

"Good sound" works and lets the playing come through without any huge blemishes.  "The Perfect Sound" is a chimera that life is just too short to spend time chasing. 

If your machine does not have an actual line-in jack, the cheap and easy fix (assuming you are using a desktop and have at least one slot open) is to buy a soundcard with a line in and pop it into your machine.  Used ones that can do a reasonable CD quality often go for less than 10$.  If you're using a laptop, there are other options that aren't too expensive either. 

But you also may be able to fix that small distortion problem you're hearing by just finding the right combo of the slider on the computer and the output knob on the preamp.  Like with everything in recording, it can take some messing around and experimenting to get everything sounding the way you want.

But to my ears, your recent efforts are sounding good!  Good job!