Fender Rumble 25 v3

The knobs make the floor resonate

 

Each knob controls a certain frequency sweep, typically more volume, use less bass to control the low end "boom"

 

Most suggest to start with all the knobs at 12 o clock and adjust to what you want to hear.
I recommend moving the amp away from you so you hear all of what is projected

 

Hi Lanzy! I've got a V3 Rumble 25 that I love. Here's a link that may help you, and be sure to check out The Fender Rumble Club as you are more than welcome there.

Fender Rumble V3 Settings?

Also lift your amp up off the floor. Put it on a chair to keep it from 'booming' through the floor.

 

Even though there is not a whole lot of information in the 'owner's manual', so read through that. In the specifications section you'll find that the tone controls are voiced at these frequencies: Bass: +/- 15dB @ 60Hz; Mid: +/- 15dB @ 630Hz, and Treble: +/- 15dB @ 6kHz.

It is important that you experiment in order to grasp what each control does to your sound.
First start with everything at 'noon', or 12 O'clock High and the Contour Filter off/disengaged. Put the tone control of your bass at about half way or a little less. Now pull your Rumble's mid range control all the way down. Play an open A, and as you play start to slowly increase the mids until you have it fully boosted. Then go the other way playing an open D.

Do this with the bass and treble controls too until you have complete understanding of what each control does to your sound.