Active Bass Circuit Issue

[DaLi]

Hi all, I'm looking for some advice about what to do with my Harley Benton active bass. It was my first bass back in lockdown and I've played it on and off since but when I came to pick it up the other day I pushed in the push/pull knob to engage the active circuit and got no sound! I then turned up on of the EQ knobs (from memory I think it's the mids) and it farted into life emphasis on the fart as it was heavily distorted and as soon as I rolled the mids back from 10 the sound cut out again. When I pulled the volume knob and turned it passive the sound came back albeit quiet and muddy (I don't think HB designed this to be played passive). 

At first I thought it was was a battery issue as I hadn't swapped the battery since I bought it but 3 sets off batteries from 3 different brands no dice. (Though I did find each set of batteries did change the sound when the mids were on full. old=farty distorted new=much noisier like tone on full through a distortion pedal noisy)

So my intuition is that it's not a battery, pickup or output jack issue and this leaves me with the circuit itself. I did a little bit of testing mids on full and playing with the other EQ knobs and you can hear the treble and bass knobs shaping the white noise so they seem fine. I also opened up the cavity an all the solder joints looked fine nothing hanging off or cracked. My best guess is possibly the Mid pot is broken? I'm kinda hoping it's not the preamp as then it would be a tough decision as to whether it's worth buying a new preamp as most on Thoman are 75% of the cost of the bass 😅

Anyway apologies for the long post just wanted to cover all the details and see if anyone had had this issue or knows a fix!