One thing that is helpful when using an amp that is not the best-get used to how some of your favorite recordings sound on that particular system before you do any mixdowns. I mean cd's or something, stuff that was mixed on state of the art stuff somewhere else. This will help you to determine referrence eq your own mixes according the the real world, not just to your amp. ALso a problem with lots of cheap amps is that the subtle stuff gets lost, like thin guitars kept down in the mix, or whispery backing vocals. you may find yourself bringing them up farther than you need. (just an example) SOOO... listen to a test mix of the stuff on other systems before you commit to the final mix.
Also watch the volume, or take regular breaks on mixdown day so that your ears don't fatigue which adds other inaccuracies to the matter.
LK