If it's still poor quality after using a good quality CD, take it back to the installer and ask him nicely to do a quick job for you to work out what's the issue.
He just needs any random headunit (that you don't need to buy), take it to the boot of the car, give it some +12V to the constant and accessories wire, run a remote wire from the headunit to the amps, and run a ground wire to the headunits ground wire. Should be easy seeing as the battery is in the VE boot.
You should now have a functioning, temporary headunit in the boot. Hook some RCAs up between the amps and the headunit.
Use the same good quality CD that you used earlier. If this sounds any better, you've isolated your issue to the headunit/line-out converter.
Edit: If it's any easier, the installer might just be able to feed the headunits constant/accessories wire into an amps positive terminals and the headunits ground wire into the negative terminal, as well as the remote wire into the remote terminal. Then it's just as simple as running a remote wire between the 2 amps (if there isn't one already) and plugging in the RCAs. Certainly dodgy, but good for a temporary troubleshooting session.