Check your nut. When I went super low (F#) it was deeply lacklustre until I opened up the nut slightly so the string (.175) sits in it properly. I had tried low F# before but had not got it to work. This was the answer for me. Take the next string up, put it in some sandpaper and slowly open the nut up. YMMV
It turns out that the bass was not actually for sale, despite eBay telling me it was. We live in a strange world. But it was exciting while it lasted. The one there is not, the chase only there is.