Hum?

If you can return it, it's easy for you. Just take it back to the store and demonstrate the hum for the tech. If you ordered online, call, tell them what's going on, and get an RMA.

Edit for more info:
It could be one of those that experiences the "less than 90-day failure rate." If it was your bass, you'd have known the issue way before a month in possibly. If you have an active bass, try running it in passive mode or give it a new battery. If the problem goes away, you were due for a new battery. I hope this helps.

 

OK Lanzy, do this: with no instrument plugged in at all turn on your amp. With all controls at noon, is it humming? If yes, how much? A little? Or a lot?

Most every amp will have some small amount of operational noise be it hum, hiss, or both. My V3 Rumble 25 is remarkably quiet. I have to turn it up all the way to detect any noise at all.

If it's not your amp, then it is something in your signal path. Could be your bass; or a cable, or any effects. Stompbox effects are notoriously noisy.

 

If it's the bass causing the hum, it's either a lack of shielding or the ground lost continuity, and since it never hummed before, it can't be the lack of shielding. Changing strings and action will not cause breakage in your wiring. It could also be the cord broke on you. But if the knobs on the amp crackle on you when you move them, it could very well be the amp. Trying the amp without a bass in it and getting no noise is so not an indicator of whether your amp is working right or not. I've seen amps be dead quiet with nothing plugged in them and still be messed up. Your best bet is to try the amp with a different bass and the bass with a different amp. Bring the cord, too. If you only have one of each, take it all back to the store and use what they have. If it is the bass, wiring issues are easily fixed. I don't think it's the bass, though...not if the amp knobs crackle.

 

Could be...if it is, it's probably an easy fix for a tech. I'd still get a second opinion.

 

For grins, loosen your strings and then check that all the screws are tight on the bridge (tail piece the strings go through). Re-tune and try again, it could be loss of ground under the bridge of the bass.

 

Lanzy,
If the humming is 'b' note (in A 440 tuning), this is likely a 60 hz hum. I've used an Electroharmonix Hum Debugger to good effect.

 

There is a bare ground wire under the bridge that grounds by pressure between the metal bridge and the paint. The change of string tension most likely made the connection go away (the wire digs into the paint). Have a guitar tech move the ground to a fresh spot later.

 

Well now that you said tightening the bridge screws helped, it does make me think your ground wire is loose under the bridge, so maybe no need for a second opinion.

 

Get your neighbour to come plug his guitar into the amp and see if there's still hum.
Sounds like a broken ground somewhere.

 

For curiosity's sake, how close to your amp are you standing?
If you are close (within 5' or so) the amps tranformer can induce hum.

 

Do you have compact fluorescent lighting in you practice room. My Ric picks CFL's up from 10 feet away.

Grounding is the right path to investigate, but EM noise from external sources will only make a noise issue worse.