There was no V2 Rumble 25, it was "upgraded" to a 30. So, yours is a V1 Rumble 25 that is from 8 to 15 years old. Any hissing noise or static could be from old, corroded connections on the outside or inside, which may require an amp tech to find and fix, if possible.
Hopefully, any other V1 Rumble owners here can tell you how theirs is doing and what they may have done to clean up the sound.
Please, DO NOT wear your headphones while plugging them in to a powered up amp! I hope I misunderstood and you're not doing that! And plug the AUX cord into your phone first, then the amp; just like like plugging your instrument cable into your bass first, then the amp. Better yet, turn the amp off while plugging in any connections. Hearing is too valuable to risk so easily.
Yes, that's how the AUX input works, even in the latest V3 Rumbles. You control the volume from whatever device you're plugging into the amp. This helps keep the amp cost down.
Auck! House wiring is really difficult to diagnose over the internet. Get a very knowledgeable person to troubleshoot that in person. Enough said.
Good luck!