Ksa 300s

Just picked up a nice one but when I got it home and turned it on it has a very slight hum coming from the speakers it sounds great when you give it some volume is this normal or should I be calling the seller it was supposed to be recapped
Check your electrical.     Mnay homes have poor AC current”
ive switched cables, etc, numerous times,
 check your incoming current!

   Turn off preamp, with ,amps on, if u still hear a staticky tweeter noise, it’s probably the. Preamp.  
 Preamp off, blackness, from speakers, probably the preamp , old capacitor, needs recapping.

 I’m breaking in a pair of Odyssey kismet amps, I have a right speaker tweeter staticky buzz, I k ow it’s the power from the house, as it disappears from specific hours of the 24 hr clock.  
 This am, my Furman shut down, from over voltage, it was two dots in the red, which was well over 125V - q130V
 it shut down the system.
 After about 8AM, it went down to 110-115V, 
shut off, restarted Furman , and all is well!

 Our line voltage is shite here

I had a KSA200s & it was dead quiet.  I could turn the volume quite high with nothing playing & it was like it was not even on.  I was using a Krell pre amp as well if that matters.  
On a sad note the KSA did fry itself after a while and I replaced with a FPB300cx which is just as quiet.  
Perhaps this will help.
Are you using rca or balanced cables?  If rca, you need to connect a jumper pin between pins 1 and 3 of input balanced connector on amp.  The negative input must be shorted otherwise you will have noise issue.
sfstereo:  That is an extremely good point that I had completely forgot about.  FWIW - I was using balanced.