I think I found out my own answer. It seems like the headphone amp not only affects the volume, but can colorize the sound as well. It's bound to be bad coming from a small boombox anyway. Well, it turns out my friend is going to go ahead and take them to that guy (without him doing any editing), but I'm still going to try it myself (because I'm stubborn!) just to see how a cheap boombox can compare to a studio quality tape deck.

M60s
VP150
QS8s
Marantz SR6003
Samsung LN52B550
Oppo DV-980H
Insignia NS-WBRDVD