You could try the amp in another location just to rule out there being local interference external to the amp. The problem would seem to be in the power stage of the amp as you say it goes away if you connect to something else using the DI output - does the noise also stop if you change the mode of the DI output to Performance Mode?
You could also try the pencil test to check whether any of the valves may have become microphonic (the pencil test refers to a method whereby you take an insulated stick like the casing of a biro and lightly tap each valve whilst listening to the speaker output - any noticeable microphonic output generated by any valve indicates the valve needs to be replaced)
However, if the problem isn't microphonic tubes or something at your location, then you should take the amp back to where you bought it and get them to verify the problem and to exchange the amp for a new one if you can - in my opinion.
+1 on trying other locations...sounds like interference of some kind...
try turning off any nearby WiFi routers, phones...ect...anything high frequency wireless...My router adds some weird things on signal if I get too close to it...Try using a power conditioner with noise suppresion if you are not doing that...