I have problem with my Tenor150 - advices welcome

i am experiencing a random waving little high frequency noise in the tweeter of one of my JBL K2S9800, that i tend to believe, comes from my Tenor 150.
Intensity is variable and noise is not always present (usually not there at start and tend to be less present after 4/5 hours).
It is not a uhm (wich is also slightly present).
It does disappear when amp is in mute mode but remains kick and fitting when the amp is in stand alone mode (with no connection to the pre-amp).
Noise has increased in the last days.
When i change the plug orientation noise is migrating from one channel to the other.
Voltage is a stable 240v (with no ground due to the age of the electrical installation)

Thks beforehands for your insights.

I did not get answers from Tenor to the mails sent in the last two months and i understand they have other priorities at the moment but i would really like to have this problem fixed.