"Power supply sounds like it could be bigger"
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Yes, it could be. But then, it wouldn't fit inside that box anymore, or maybe on the desk. Both of which were considerations when it was designed something like 30 years ago to compete with the 'other' similar amplifiers on the market. None of those 'old' amplifiers were designed to be the "end-all" of power makers. They were designed to give a sort of moderate, but long-lived service, which they have (they'er still around, ain't they?).
- 'Doc
PS - One little known fact that isn't spread around much is that any thingy that produces much power tends to drop the stray electron now and then (usually fall to the bottom of the thingy's case). Those electrons do have mass. After heavy use, the desk/table it sits on tends to creak a bit. Now you know why...