Anyway, just an update. I got my problem solved, together with some interesting findings where i think most can benefit from it.
The problem i described above, only seems to happen in my room where the power socket/outlet to be suspected to be not grounded properly.
Problem: Besides the noisy treble eq knob issue that i encounter, i am getting all the general issue with the DT50 amp like all other user and owner out there where issues like occasional pop, hum, hiss and really loud noise, all i believe is due to the grounding issue of the power outlet.
Findings: So what i did was i brought the amp out of my room into the living room, where i believe the power outlet to be of better built and ground. Before that, i also swap and interchange the position of the 2 unit of 12AX7, making sure it is seated proper. Next up, i power up the amp, plugging my guitar direct without any pedals running in the front. And voila! True enough the DT50 was quite like a mouse. It was soooOoo quiet to the extend where i can even safely say that i can do proper recording through it, without even needing to 'gate' it like some Line 6 tech suggest. Mind you also i did not change the two EH 12ax7 to other brand.
Next, i boost the volume and master real loud and start turning the treble eq knob, and voila! No more rotary like dialing sound.
Next, i brought the amp back into my room and every single problem just came back. Thank God i have another power outlet in my room where i can use, so i plug it into it. And behold, there were a noticable hum (definate noisier than when in the living room compared) but it is still ok. No treble eq knob rotary like sound issue as well.
Next, i plug my guitar pedals and run it infront of the amp. And the problem came back. The noise (much quieter this time since the amp is at the new power outlet), and also rotary eq knob sound is back!
At this point i start to suspect it's the grounding issue on my pedals as well. I have a KFK 10 band eq pedal on my pedalboard that runs on 18v. I ran a OneSpot adapter for the rest of the 9v but at one of the daisy chain 9v plug, i placed a diago 9v to 18v adapter to power my mxr kfk 10band eq. I suspect this contributed to the noise and general grounding issue that can lead to the amp to behave weird as well, hence i unplugged the 18v KFK from my chain and VOILA! No more noise (small hum is still there, i suspect due to the power outlet again, but at least much better than the other outlet in the room), and also no more eq knob issue!
Solution: MAKE SURE YOUR POWER OUTLET GROUNDING IS GOOD! And don't plug multiple electrical appliances into the outlet and share it with the tube amp and also pedal board power. Also, avoid using cheap extension electrical cable. Make sure it is the 3pin version with proper grounding and the cables are thick and of high quality built.
I guess tubes are more senstive since it's microphonic. And since the DT50 preamp section is still running digital (boosted level by the 12ax7), i suspect any grounding issue can really magnify the issue, since digital can be sensitive as well at times. The power supply can really make or break your amp sound! =)
Hope this helps the rest.