I've seen two different versions of the Rumble 25. One has the classic Fender grill cloth and the other has the metal grill guard. If you have the grill cloth version, your Rumble 25 doesn't have an effects loop and you will you have to get a pedal or A/B/AB switch to get that sound into a higher powered amp and run that into the effects-in of the second amp.
If yours is the black Rumble 25 (newer I presume) then you don't need any stomp-box or external switch because it has a built-in effects loop. Presuming you prefer the tone you get out of your Rumble 25, what you want to do is run the signal from the preamp of your Rumble 25 into the power section of another amp or combo. That is easily done by running an instrument cable out of your "preamp out" jack in the front into the "effects-in" of another amp. This is called "slaving" an amp. It bypasses the preamp of the slave amp and hooks the master amp's preamp into the power section of the slave amp. Then any EQ changes made in the master amp (your Rumble) will also come through the slave amp. The only thing you need to do with your slave amp is adjust the master volume so you get the balance of sound you want between your two amps. If you change your volume on the master, you will still need to adjust the volume on the slave but EQ is carried over because you're using the preamp from your Rumble 25 for both amps.
Now, If you want to run both amps at once you may need a simple 1/4" Y connector (female ends all around). Most amps that you run a preamp-out (usually called effects-out) won't continue the signal to the power section of your master amp unless you plug the preamp-out back into the effects-in of your amp. Some will continue it to your first amp without interruption. So just try it and see which yours does. If your Rumble 25 speaker goes silent when you slave the other amp, just use a Y cord on the preamp-out and run one cord to the effects-in of the other amp and the second cord into your the effects-in of your Rumble 25. Then you'll have sound coming out of both amps.
I used to slave a GK250BL form an Eden Nemesis RS400. Was pretty cool to hear the Eden tone I liked coming out of that entry level GK (IMO not up to the higher quality GK gear) running a separate speaker cab. One of the cool things that most Eden amps would do was slave an amp by running a line out of the headphones jack into the effects-in of the slave (none of the Eden WTX series class D amps can do that). The neat thing about running it out of the headphones was that it gave a hotter signal to the slave PLUS, when you turned the volume up on the headphones, it also turned the volume up on the slave amp. So you set it up and balance the volume mix once and then everything is controlled with the master amp including volume. I don't see anything on the manual for your Rumble 25 that says you can do that, so I wouldn't recommend trying it until you spoke to someone at Fender about it, just to be safe.
If yours is the black Rumble 25 (newer I presume) then you don't need any stomp-box or external switch because it has a built-in effects loop. Presuming you prefer the tone you get out of your Rumble 25, what you want to do is run the signal from the preamp of your Rumble 25 into the power section of another amp or combo. That is easily done by running an instrument cable out of your "preamp out" jack in the front into the "effects-in" of another amp. This is called "slaving" an amp. It bypasses the preamp of the slave amp and hooks the master amp's preamp into the power section of the slave amp. Then any EQ changes made in the master amp (your Rumble) will also come through the slave amp. The only thing you need to do with your slave amp is adjust the master volume so you get the balance of sound you want between your two amps. If you change your volume on the master, you will still need to adjust the volume on the slave but EQ is carried over because you're using the preamp from your Rumble 25 for both amps.
Now, If you want to run both amps at once you may need a simple 1/4" Y connector (female ends all around). Most amps that you run a preamp-out (usually called effects-out) won't continue the signal to the power section of your master amp unless you plug the preamp-out back into the effects-in of your amp. Some will continue it to your first amp without interruption. So just try it and see which yours does. If your Rumble 25 speaker goes silent when you slave the other amp, just use a Y cord on the preamp-out and run one cord to the effects-in of the other amp and the second cord into your the effects-in of your Rumble 25. Then you'll have sound coming out of both amps.
I used to slave a GK250BL form an Eden Nemesis RS400. Was pretty cool to hear the Eden tone I liked coming out of that entry level GK (IMO not up to the higher quality GK gear) running a separate speaker cab. One of the cool things that most Eden amps would do was slave an amp by running a line out of the headphones jack into the effects-in of the slave (none of the Eden WTX series class D amps can do that). The neat thing about running it out of the headphones was that it gave a hotter signal to the slave PLUS, when you turned the volume up on the headphones, it also turned the volume up on the slave amp. So you set it up and balance the volume mix once and then everything is controlled with the master amp including volume. I don't see anything on the manual for your Rumble 25 that says you can do that, so I wouldn't recommend trying it until you spoke to someone at Fender about it, just to be safe.
