Headpieces don't always reflect neck profile but in this case it very much does. It's not excessively deep but rather full round the sides (Warwick did similar necks after their baseball bat period).
That (as others have said) is the problem. The audio interface side of it will be latency free but anything using MIDI will have a slight delay, which changing ASIO parameters won't alter.
Bass -> multi-FX (or first effect chain) input
multi-FX external loop send (or out of last pedal in first effect chain) to amp input
amp effects loop send to multi-FX loop return (or second effects chain)
multi-FX (or second effects chain) output to the amp return
The first part of the chain precedes the amp tone shaping and anything else in the amp, the second part follows it.
It looks to me like there are two candidates for the black wire (the two that @sandy_r has highlighted) and two candidates for the white wire (the two a couple of cm to the right of the first two). That's going by the relative length and positions of the black and white wires.
https://www.sweetwater.com/ - massive USA online musical instrument shop. Searching for gear generally brings them out somewhere near the top. They seem happy to give advice on a wide range of subjects so you could try contacting them. And have you tried https://www.hartke.com/contact/ yet?
I'm another Schaller user so I can't give a definitive first-hand answer but I think someone has mentioned the rubber plumbing washers from B&Q before. https://www.diy.com/departments/plumbsure-rubber-plumbing-washers-pack-of-2/184672_BQ.prd
Someone ( @ped ?) organised some Basschat special ones ages ago but I suspect they'll all have gone now.
It was McDonalds. In my defence, the drummer and I were both hungry. Anyway, I only played one bum note and the guitarist cunningly worked it in. http://tauzero.co.uk/music/mp3s/TornInTwo/I'llForgiveHer.mp3