dont think negative, to be honest i wouldnt worry about warranties because pioneer doesnt break for years so go ebay if you have to and dont worry , i highly recommend p580mp its cheap and sounds like p7800mp because it has the great eq-ex like sound field control and youl like the effect
the 580 is the Premier model. the 5800 is the non-premier model. the 5800 won't support ACC files, which are the ITunes. the 580 will.
i've also heard the premier models have higher quality components and better build quality, but i can't back that up.
the 580 should not sound better then the 7800 unless the person using the 7800 didn't know what they are doing or it was hooked to poor quality equiptment. the 7800 or 780 is a much better unit. way more EQ and X Over options, higher votage pre outs, 24 bit burr brown DAC vs the 16 bit with the 5800....much better over all.
the 680 has more display options and more EQ and X-Over options. the controls are different too. it has the rotary commander vs the cross keys. not totally sure how much that affects things, haven't messed with both in person to compare.
id go fo 5800mp its cheaper and you will enjoy the built in eq-ex(equalizer extra, gives you different sound effects as if you where listening to a live performance, but the 680 doesnt have it but better face and cross over