I have an old Gallien Krueger 2000cpl preamp (think 80s Rush/Iron  Maiden).  I'm trying to figure out the best way to get this thing hooked  up so it doesn't sound like nails on a chalkboard.  Ive never used  anything other than a combo amp so the concept of a preamp is foreign to  me.  It has a headphone jack on it but it just sounds scratchy and  terrible (i believe this is well documented).  I tried taking the line  out and going to a GCS-2 cabinet sim and it takes the edge off a good  bit, but still just doesn't sound good.  

Here's a pic of the outputs on the back of the preamp.  

This is a pic of Alex Lifeson's rig with a similar preamp (a little more complicated then I would like to get.
What's the best way to get good sound out of this?  Do I have to buy a  cabinet of some sort?  My cheapo marshall combo amp doesnt have any  input outside of the guitar input.

Thanks for the help
You'll need a three least a power amp and a cab
Alternatively you can run the line out to your computer and add some impulses and eq.
Jumping on dat gear sig train.
PRS Hollowbody II / BKP Warpigs
Strandberg OS6T / BKP Aftermath
Strandberg OS7 / Lace Poopsticks
Skervesen Raptor 7FF / BKP Warpigs
Skervesen Raptor 6 NTB / BKP Juggernauts
Hapas Sludge 7 FF / Hapas Leviathan
Anderson Baritom / Motorcity Nuke BKP Sinner Anderson H2+
Warmoth Baritone / BKP Piledriver
Ibanez Rg2120x / BKP Nailbomb

Blackstar ID:Core Beam
That must be a truly ancient Alex Lifeson setup. Not that there's anything wrong with that!

I'd play around with the Line send or fx send. You could try plugging that in front of your amp, with the amp set as neutral as it will get, or you can try to mimic a live rig live Alex's in your DAW.

Off the bat I'd say play around with IRs and some amp sims. You can try GuitarRig free as it has a farily neutral sounding amp and a cab built in plus some fx you can put in there. Maybe try one of the Thermionik sampler amps, the Plexi or early Blackface sims.