GAMEBOY!!!
i don't know, i have mixed on a million different speakers, amps, and rooms... i would have to say that the gameboy speaker sounds like it will be better for mixing on then on 3 different pairs of consumer speakers all going on at the same time... sounds to me that it is over coloring your perception of the mix just a lil bit... but yeah, you should get the other one fixed at least... and then, maybe try using one set of speakers at a time and reference between how each pair sounds and find your fav to mix on, and then, your fav to switch to as a reference. Writing is another story... hook up as many as you want... but mixing... it doesn't work... and yeah, a new sound card would help you out allot... and that cassette deck thing is nothing but trouble... i have done that a couple times while i am traveling on the road and need to monitor some stuff off my laptop or sequencer onto speakers and its the last resort... take it easy bro... and good luck