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Its 12VAC makes a 15VDC supply. You are feeding maybe 12.6VDC into the full-wave-bridge rectifier which produces a supply of only 10.6VDC.
Originally its output was 14W per channel but now is only about 6W per channel.
Why did you buy a charger? what powers the charger? Doesn't your van have an engine, an alternator and a 12V battery? Doesn't the van have a lighter socket? Then why didn't you simply buy a lighter plug?
You said that the speakers use a 12AC power supply. Inside the speaker, the full-wave-bridge rectifier converts the 12VAC to 15VDC.
If you feed 12.6VDC from the van's 12V battery then the rectifier reduces the 12.6V to 10.6VDC. The rectifier voltage drop depends on how loud the speakers are playing and is about 2V at full power.
If it's a charger then it might have a current limiter inside. Result is low current. Heavy amplifier units require high current to output it's full power. Yours 14W stereo looks like there's a 2A current requirement. If it's the case then you are simply not getting the full punch from T20's like AG told.
Anyway just tell me if it's a worthwhile buy in terms of comparison with other desktop speakers ?