Had many of these amps, speaker upgrade is great. Another mod nobody seems to mention is an upgraded reverb pan. I have a new one sitting around in its box if your interested, they are not expensive and found on amazon, Mod is the brand.
New pan makes a world of difference with the reverb.
Oh and if you get tired of the boxy sound out of the combo, plug it into an extension cab, you won't want to go back
That is an amp that responds well when you roll some ANOS tubes in there. It loves Mullard or Telelfunken for power tubes, but it eats them. I'd look at a bias reset using better tubes. But the stock tubes were very bad sovteks.