Ok, I got your PM, but figured I should reply here just to clear it up....
First, "hum"..... You mentioned two hums, one a "hissing hum" ONLY if you turn the amp up to 6 or 7 on both gain and "volume" (Master, I assume) with nothing plugged in ..... AND a different hum when you have a cord/bass plugged in.
Now, there is "hum" and there is "hiss" and there is a hum that is "hissy".
Hum is deep with a good amount of lows, with a definite "tone" or "timbre"..... "Hissy hum is like that, except that the lows are reduced, and what is left is all the harmonics... you can tell it has a 'tone" to it, but it is a "buzz", there isn't much if any real lows to it. "Hiss" is just that, with no particular tone to it, although setting the EQ can change it and almost give it a particular tone....
What I am getting here is that the turned-up /no cable hum is the softer, buzzy hum, and the one with an instrument is LOUD and has some solid lows to it, not just buzzy or hissy.
If that isn't the case, tell us.
While I'd like to see "zero' noise even if the amp is turned way up, I'm going to separate these two somewhat different "hums" and I am going to say the hissy, buzzy one when turned up is "normal".....doesn't mean I LIKE it, but I *understand* that one, could be tubes etc,etc. it isn't really unusual, and generally depends on EQ/tone settings etc,
So, lets concentrate on the one that occurs with something plugged in.
I very much doubt any power cable is going to do anything... you already found that out.
There are several usual causes for this kind of hum, that goes away when you touch or get near to grounded things, like the amp.....
In general, it means something in the bass or the cable or the amp input jack is not connected, broken wire, bad solder joint, loose screw, etc.
if you can touch the guitar strings and it goes away, "usually" it means the shield over the controls/wiring in the bass is not connected, and hum voltages picked up on your body are getting into the circuit. The strings are usually grounded through the guitar cable. When you touch the strings, you are then "grounded" and no longer have any hum voltage on your body, so no more hum.
That usually ALSO works if you touch the amp chassis, maybe even if you touch the screw on a light switch cover, or a radiator in the room, or nearly anything else that is grounded.
OK.... you did say you can touch the strings and it goes away....
BUT....
Usually that is with a particular instrument..... but you say it is with several..... Hmmmmmm
If the CABLE is bad, one of the two wires is usually broken.... if the center wire is broken, you get no sound, or weak sound....
If the shield is broken, you get hum..... LOUD hum..... but usually touching the strings usually does NOT work.... although touching the amp often DOES work.
OK, but you say various different cables all are same..... Hmmmmmmmmm
If the input jack is not grounded right, maybe broken solder etc, it is pretty much the same thing as with a bad cable..... Same things, and usually touching the strings does not do much if anything. Touching the amp chassis often WILL cut the hum. And maybe it is OK sometimes if the way the cable hangs jams the bad solder together and makes contact.
The problem I have with giving an answer here is that what you say is not consistent with ANY SINGLE one of these,........
It's just like a bad cable or jack, but you say touching the strings cuts the hum.
It's just like an unshielded controls problem, but you say it does it with several instruments and cables.
As I mentioned, it may be some odd cancellation thing..... although I admit I can't come up with a good reason for that.
Just to straighten this out..... can you tell us FOR SURE what "kind" of hum is in each case....?
Is either one a deep, bassy hum that is pretty loud?
if so, which one?
And when you touch things, or switch instruments / cables/ whatever, does it stay the same?
If I am way off on what I am "getting" from your information, correct me and let's go from there.
I don't want to send you off to a tech without a good reason, because it will probably just cause you money and time if the problem isn't one that needs a tech.....
If you don't have contact cleaner, you can just run a plug in and out really fast a few times in a row.
