The Rumble 15, 25 & 40 differ from each other by the contols, and tone flexibility, they have.
The Rumble 15 has only 4 basic controls for Volume, Bass, Mid & Treble. It does have an Aux In and Phones output and that's all. It's the bare minimum stuff for a bargain basement price.
The Rumble 25 has a couple of extras above the 15. In addition to the basic controls for Volume, Bass, Mid & Trebke, it has an Overdrive button and a Contour button for more tone voicing. When the Overdrive is on it adds distortion to the tone and increasing the Volume control increases the distortion. When the Contour is on it provides a mid-cut "fat" tone for various uses, including low volume practice, but there is no adjustment for it. You can use either, or both, buttons as you like. The only I/O is Aux In and Phones out.
The Rumble 40 is the smallest Rumble amp to have the full controls as the larger, more expensive, Rumble amps, adding a Gain control, Vintage voicing button, Drive & Level controls for the Overdrive, replacing the Mid with Low Mid & High Mid controls, and a Master volume control. The Aux In and Phones out are moved to the back where there is also a Footswitch connection (footswitch not included) to turn the Overdrive on & off, and a balanced XLR Out with Ground Lift switch for connecting to a PA or recording system.
I hope this overview can help shoppers decide on the model they want/need.
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