The S is supposed to be 60/138 watts.
It is. It gets the extra power from having a solid state rectifier.
8/4/2 ohm switch which is something i really like/wanted so i can add some cabs and bring her out once in a while. Prior owner changed the tubes to 5881,
If you're getting plenty of volume and good tone, don't worry about it. Very few techs actually set up old tube amps to put out rated power because their perceived volume is generally what people are looking for. True 5881's will reduce output power somewhat, but if they're Sovtek, that may not be the case as the tube they market as a "5881" isn't and is much tougher than the real thing.
and some notes below A cause a rumble at any decent volume. Not sure yet how to attack that...
Those are the electrolytic caps literally rattling. Old caps can still hold value but dry out to the point where they'll physically vibrate internally and pass AC surges at the vibration frequency. I've seen it in tons of B-15's.
It weighs over 100 pounds with the current square magnet speaker. If i upgrade it may be unmanageable
That's a CTS, and they get a bad rap undeservedly. Leave the stock speaker in the amp. Experiment with the new cab.