I'm confused you actually LOSE voltage from your radio being on standby? .... Buddy. That's just weird, even my mids/highs cranked up don't drop my voltage at all.
With the ignition and radio off the only current draw should be the memory and clock. I'd be suprised if that's any more than .01 amp but it's not usually in the specs. More common power drains are lights that you don't know about, like glove box or engine compartment.