Here's an idea:
Connect your x-track to your stereo. Now listen to a CD or Radio. While playing your song through AUX, switch back from CD/Radio to AUX to CD/Radio and back, until your signal from AUX (your x-track) has the same general sound level. KEEP THE VOLUME THE SAME WHILE YOU ARE DOING THIS. Now record your song for a bit. Listen to it off the tape. Does it sound about the same level as when it came in through AUX? If not, try these three things...
1) Make sure Dolby is OFF. Some people like to use it, but I find it just chops the highs out. Even w/ Dolby, you're going to get tape hiss. Disable it for a bit.
2) Make sure you've cleaned your tapeheads. If this is your standard-issue WalMart stereo and you've used it for a while, the heads will need cleaning. Don't go out and buy a cleaner--just take a Q-tip and splash/dip it in medical alcohol (isopropyl alcohol). Open your tape "door" and rub the Q-tip on the large metallic looking thing in the middle of the "hole". (I'm not trying to be condescending--you just might not know where the tape head is, you know?) Let it dry for a while, "repeat if necessary."
3) Use High-Bias tape. You probably won't be able to use Metal tape, so avoid it. Even if you can, it tends to do *something* to tape heads that aren't designed for it. But you don't want normal bias, either. I think High-Bias is either Type II or Type III. So, don't use Type I or Type IV. This will allow for greater saturation of the audio on the tape media itself, and allow you to turn your song up louder as it is put into your stereo's AUX.
hope this helps...