The realist is a very good pickup on resonant-topped carved basses, but on a (particularly newer or thicker) laminated bass it can be less than impressive. The bass you are using with it, coupled with the gut strings, is a recipe for dark, indistinct sound - as you have found.
Consider a pickup further up the bridge, which will get more "detail" from the strings and less "boom" from the body. Something in the wing, or in/around the adjusters (if you have 'em).
For instance: A K&K Bass Max
, Schatten RB1
, Fishman Full Circle
, Realist Lifeline
And if you're rockabilly (or slap)-specific, consider a "clicky
" pickup as well. K&K
, among others, offer two-element systems which provide both a "main" pickup and a "clicky" pickup - and the means to combine them and tweak their tone (usually in the form of a compact two-channel preamp
.) There are other choices as well, I'm sure some others will chime in.