Amp not enough power

Thanks, everyone, especially Al. I had suspected about the power Caps to my Aragon amp. They are the direct voltage supply to both channel boards. I will try to locate the caps.
You can treasure a product and still keep it going unless something major breaks. I will certainly try to keep my Aragon in good working order.

400W is only 1.5% louder than 380W - don't worry. I would recap this amp for better bass control. I changed power supply caps once and that is what I noticed. Bass became shorter more dynamic and cleaner.

Older caps have higher ESR (internal resistance) because they dry out. This internal resistance will keep increasing progressively because of increased self heating (but it will take many years). Eventually it will become so high that full power test, that you did, might trigger thermal runaway (temperature increases ESR that increases temperature by self heating even further and so on) and cap might blow up its mechanical fuse (rubber plug) or explode.

Electrolytic caps are the only components that age and are worth replacing if you like the amp. There is a good chance that you might be able to find today much better capacitors (lower ESR) than original.

you are so close to meeting spec and you'll never whatsoever discern the difference of 20 or so extra watts, even if you do achieve that.
Now if you want to tweak it for enhanced sound quality that that is a different motivation, and I wouldn't stop at just the power supply caps alone.

120V isn't input voltage...

First of all, are you certain the power rating has decreased over time? Have you had the same measurement when you first had the amp? Like someone else mentioned, it's within 5% of the rating. Maybe your amp was assembled that way.

Power rating is not like the maximum speed of the car where you measure the speed with full throttle. Power rating is measured *as relative to the distortion*. As the power goes up the distortion goes up after certain point. Are you sure you are using the same distortion percentage for clipping as published spec?