input voltage question

hi, someone the other day has offered me 500 bucks for alpine 9887 HU and since i only paid 300 for it, 4 months ago i kinda of am considering it. now though here is the problem, i want to sell my car in a couple of months and the car i am looking at has a double din opening and i would like to take advantage of that and get a double din in dash dvd player so just buying another 9887 and just keeping the 200 bucks different really isnt a option. now what i was thinking was this. i have an mp3 player that i can directly connect to my amp. ( sundown sax100.4) but the thing is when it is directly connected to it i can only get the voltage on the output of the amp to 4.71 volts not the 20 volts i want 100rms @ 4 ohm) so i am wondering will a line driver fix this cuz i would rather spend 40 bucks on a line driver then to by another head unit ( and i really dont listen to cds and all my cds are mp3 cds) and the sax100.4 has the x overs on it that i need and my mpe player has some basic equazer stuff on it. or does anyone have another way of fixing this.

the audiocontrol 4.1 has an additional input and a volume control .... problem is, it's pricey

well i can use the volume control on the mp3 player now is a line driver wha ti am looking for because i see that, that has one build it. cause it would be cheaper for me to get a line driver and then something like this.

http://www.woofersetc.com/index.cfm?fuseaction=product.display&Product_ID=1294

most line drivers I have seen (the old school ones) were a fixed amplification factor, they did not have a volume or sensitivity control on them.

so i might just be better off going to walmart and getting a lower level pioneer that has aux input level with input sensity then just doing that.

hard to say because I know next to nothing about todays line drivers.