Depends on what kind of HU you're looking at getting.  I'm never a fan on factory audio.  But if you're going to get some cheap HU then you might be better off sticking with your current HU. 

yeah its possible but it'll cost you more trust me, I was gonna do that until they started talkin about the other stuff that would need to be inolved with doing it, like installing a seperate equalizer, bass control etc.  so your better off buying a new (good)  deck and having it all installed at once or install the deck yourself and save some cash to get your amp hooked up.

if you buy an xtant amp you don't even need the high to low level converter, it'll filter it at the amp

I hooked up a fosgate p2 and an xplod amp to my factory hu, and it worked great.  I bought a new hu cuz i can now turn my sub on or off, whatever you like is what you should do.  I just didn't like my sub running constantly.

the hu is a pioneer that i bought at best buy.  it was about $220.  i thought it was well worth the money.  now i just have to get some aftermarket speakers cuz i blew my other ones

all right, sounds good.  now i need the money to do that.  school is takin it all away from me