[Analog mono preamp -- Dub Sound System]

Greetings !!
I have not any particular knowledge in electronics more less in analogic world.
I'm trying to troubleshoot issues with hand made sound system (four stages sound system with subs, bass, mids, top).The main issue concerne the preamp (working as 4way crossover and 8 bands equalizer) that is expected to be connected to the amps directly with its 4 unbalanced (mono) output.
Some monthes ago, and the one who did this never intervented to fix it up, the preamp was used in a "hostile configuration" and suddenly the sound collapsed as we reduced the volume from 0db to -6db (as someone who heard the phenomenum and told me, it was when someone released a smartphone charger which was connected on the same plug as the preamp that was working) ...
Since this time I "lost" the input gain (labelled vol at the front panel but this is clearly the input gain for the selected input, here a turn table). When this preamp came back from repair a couple of monthes before being sabotaged a second time, when we put this "vol" button at 9 o'clock we got sufficiently powerfull signal to "illuminate" most of the leds for the four stages. Now if we cross 9 o'clock we got additional interference sound like "white/pink noise" and there is no "modulation" regarding the leds at each channel :{
We have also buzzes on the 10 inches used as highmids that never appeared before (I think the turntable is involved as when I cut the phono switch to line this ground sound dissapeared instantly).The "noise" heard follows the main volume (its intensity rises) we could only cut it with noise gate at -50db/-45db but it cuts also some frequencies we want to hear in the music we play :{
This noise appears only if we set the vol knob at a position over 9 o'clock (one portion of a millimeter to the right is sufficient to hear it suddenly appear).
I heard that most of the preamps have a "6 db resistor" to attenuate the input level... do you think this resistor may be shunted and continuously apply the 6db signal attenuation ?
Do you think the knob itselfs could play a role ?
What can I say to the friend I contacted to efficiently work on this preamp regarding the descriptions of the phenomenums observed ?
The knobs on this preamp are all the same and each time we set the knob more than 12 o'clock or 3 o'clock (except for the 8 band EQ that doesn't seem to be affected by this issue) we got this "noise" appearing suddenly from nothing.
To be precise: imagine the volume button is a scale of 1 to 10 with 10 subdivisions between each value, when I set the button from 5 to 5.1 I got the "noise", from 1 to 5 no noise, from 5 to 5.1 I got the noise.
I have a line isolator but I don't know where to put it, before the input or after the output just before the amps ?I think I have to buy another one, for now I use it for the trebles and highmids stages (lifted not grounded).
I also have a DI Box to use with the turntable but for these last tests we did not used it.
The four amps (in reality there is 7 amps with 6 amps bridged 2 by 2) are connected directly to the preamp.We have insufficient input levels at the amps because we can not rise the gain more than 9 o'clock at the preamp; so we have to push them high --> that doesn't help the attenuation of the "noise" generated somewhere between the turntable the input phono plugs, the gain button and the output volume knobs ^^
When all is set to zero, in gain purposes (at preamp and amps) we have only a "noise floor" hearable when puting the ears straight to the beymas, no buzz, no ground buzz... when the preamp is set to just a little more than 3 o'clock -> boum !! noise.
What do you think, should I trash the preamp ?