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The pack's rating is the total output of the pack. A 3200 Ws pack will output up to 3200 Ws, total, each time it pops. That can be up to 800 Ws to each of 4 heads, or ~1067 Ws to 3 heads, or 1600 Ws to 2 heads, or so on.
The newer heads are indeed identical with the only difference being the preinstalled tubes; I'm not certain about the older ones.