Thanks for the input! Really appreciate it...

Have to check the truss rod and neck adjustments/shape because of the rattling...the fretboard seems to be flat...never sanded fretboards before and I am a bit afraid of it, for it will destroy the finish (white binding painted black and sealed with transparent lacquer...) but on the other hand it is still an instrument and should be playable and not being just an object...so I have to play around a bit, but actually not much to adjust at this stage...maybe giving it a bit of rest and tehn move on, but I already looked up for the sand block...haha

As for the jack, I had connected the shortest one, so it should be right? Just the description I found on the internet was reversed, for the shortest it sayed battery and middle one for hot. My problem is, that I like the barrel jack visually coming out of the body. That is the only reason I used it (also on the IB-4).

And about the loudness of the bass...I think I have to reinstall the humbuckers, for atm there is no way for them to come out more out of the body, it is already tight. And if they should move more to the strings, I have to make the adjustment.
The IB-4 is actually passive P-Bass with diMarzio pickups, maybe they are just louder, thats why it seems more silent on the HB in comparison...

So, the work is finished, but it also goes on Have to make some marks for the frets on the side ans move on with measuremnts and adjustments