The closest woofer is going to be [Invalid or Expired Link Removed]
I think by the time you are done with finding and swapping out a speaker, you'll have spent more money and time and have an inferior (or at least unpredictable) sound than if you had sold the old cab and bought a Rumble 15. or even a used Rumble 15. I'm a tinkerer, builder and tweaker, and even so, I'd buy a new rumble 15 rather than trying to mod the old one. It's just too unpredictable. buy one from GC and if you don't like it, return it.
This is true. A brand new Rumble 115 cabinet is $300 and contains the exact same woofer as the Rumble 200 combo. The Eminence Legend CB-158 woofer is $135.
Thanks for your replies.
In fact as my other cab is almost the same volume as the Rumble 200 one, I was thinking of getting the same speaker than in the Rumble 200. You see ?
If volume and tuning are good, the cabs should sound almost the same, no ?
I was just thinking on a cheap alternative

Buying the Rumble wasn't scheduled, I had to replace my dead amp. And prices are higher right there in Europe.
I don't really need the extension speaker now, just projecting for outdoor gigs when sun will come back in a few monthes.
Playing with my new Rumble, I got better tones from the overdive (I was aiming for just that touch of grit/dirt) by lowering the gain. It sounds nice

The eq is nice too. Seems to be well voiced to push the mids when needed or reduce the bass in a boomy room.
I don't know what to think of the Vintage setting. It sounds nice with the bass knob reduced a bit. Have to try it in a band setting to know how I like it.
What sounds strange is that the phone output has a strong Cab Sim. When I say strong, I mean it sounds way more scooped than what I hear from the speaker. Not a problem, just need some eq adjustment when playing with headphones.
As I couldn't test the DI, and as I see on the schematic in the instruction book that there's a Cab sim before the DI too, do you know if it is as strongly voiced ?
On the schematic I can see too that there's a built in HPF which is great.