I bought the Rumble 115 cab to connect to my 200 combo. The difference is surprising. The sound quality is different and much improved, even at low volume. The sound of the combo by itself would begin to degrade slightly when the Master passed 9 o'clock, got ugly past noon, all this with the gain off. Sometimes I would open the gain to 9 o'clock to increase oomph, which it did but with an unpleasant effect on the sound quality which became increasingly tinny with gain.
With the cab, the sound is much more natural, more "acoustic" and less "electric" if it makes any sense, like playing through a giant paper cone. Deeper, richer, warmer. Increasing the gain and/or master increases the volume without changing the sound much. Very happy and pleasantly surprised. I think I understand that increasing the watts, which goes from 140W to the full 200W with a cabinet connected, does not necessarily increase volume, but allows the sound to retain its quality with volume increases. If ever I need more, I would consider buying the 500 combo, which I could also connect to my 115 cab.
If this helps, I play an Ibanez RB 960 (bought new in 1985) with active pickups. With the 200 combo alone at low volume (gain 0, master 9 o'clock), the bass sounded like a jazz bass (which it is I think), electric, metallic and zingy in a good way. With the added 115, it sounds more like a precision bass, boomy, punchy, woody.