Actually, I didn't remember offhand exactly what the specs were on a Hitachi HA 1392 as I don't have any big Sharps anymore. Upon inspection of our fine service manual library, I found it's spec sheet, though, and under various voltage supplies it has different output wattage ratings (all RMS.) It's rated at 4.3 wpc on 12 volt power and a whole 6.8 wpc on on 15v power. I think that second number is probably where the 28 watts comes from. In reality, you could say it has the potential for 27.2 total watts combined from all 4 channels, I guess.
I would be more likely to believe the Sharp service manual's claim of 4wpc for the outer mid-woofers and 3 wpc for the super-woofers, all RMS. That's a little more believeable under real world conditions,