I have a 1990 Ford Mustang LX Convetible V8.

I need to replace the head unit that got ripped off. A Pioneer DEH-3000.
I've got 6 PPI speakers 3.5, 6.5 and 5x7. The 3.5 were running off the headunit while the others were running though a Power Acoustik 4x400 amp. I also have a MTX T8104A running off of a Power Acoustik 2x400 amp.

1 My 3.5s can't take the power that and they start to fart as the voluem gos up. Should I run them off of a kenwood 2x50 amp I have lieing around? If so, can I just run the RCAs in to one amp and than another without hurtring my sound Quality?
2 I was running 2 pairs of Pre-amp out puts 1 to my speakers amp and 1 to my subs amp. My larger front and back speakers are all running off the same line. Would it be worth my time to buy a headunit with 3 pairs of pre amp outputs so that I can have better sound quality and control?
3 My speaker amp is set to high frequency. Should I set it to full or leave it at high?
4. Anyone have an opionon on a good 10inch sub that fits into a small truck box with out blowing to easily? Mine is shot to pieces.

for my headunit I would like to get something a bit nicer than the prior Pioneer. I want to have the ability to play MP3s, CDRWs and maybe 3 pairs of pre amp outputs but at least 2 probably 4v not 2v. I would like to be under $300 if possible. I loved my pioneer. Any and all sugestions will be greatly appreciated.
Thx for your time,