Why the fan based translations for FFIV and FFVI suck?

Bullshit. Go play Chrono Trigger DS. If the richness & fidelity don't make you cream your pants, you are not human.

Definitely, they got CT right, although if you listen carefully you'll notice immediately they added some distortion to the bass, leving other instruments inchanged, probably so the bass could be heard better on the small DS's speakers (small speakers are unable to reproduce low frequencies, so adding distortion adds harmonics which are heard).

Another difference is that the echo is different but that's another story, and a hardware limitation of the DS which can't natively do true echo like the SNES so they have to simulate it using remaining channels to echo the main channels. In CT they did this work very well they simulate the echo from the original SNES version very well, I can't say the same for the GBA FF remakes.... This is one of the big difference in my FF6 patch where I use true echo, yet I wasn't able to make it sound exactly like the SNES' unfortunately I don't think it was possible using Nintendo's sappy sound engine.

Then it all depends to the designer's choice, either they care about the small speakers and do stuff so that the music still sounds the same through them, or they don't care and assume the user to use headphones. Golden Sun is a perfect example of the latter : You miss like 50% of the music if you play the game trough the raw GBA as you can't hear the bassline at all !

PS : Oh and I play the trumpet too but I'm used to synths which are unable to emulate correctly brass instruments so I still think you can recognize trumpets, even if it doesn't sound like the real deal.