Where can I buy a fairly good stereo (pref. CD but Cassette ok) in 6v?

Try replicas of old VW radios, they were 6 volt - pre 65.

If you're talking about the pre-amp out, the voltage really doesn't affect the quality of the output so any voltage is fine.

This is because the gain or level is used to match the input sensitivity to the amp in volts.

Here is a guide that will help you set the gain correctly http://www.datafilehost.com/download.php?file=6d26...

You'll need a multi-meter (AC voltmeter), Microsoft Excel and a way to burn an audio CD from an MP3.

If you don't have Excel, e-mail me and I'll send you a condensed version sparky3489@yahoo.com

See my site for more info http://spkrbox1.spaces.live.com/