When I played in the 70's I had hard calluses on my plucking fingers. I no longer have those, but I miss them because they give my notes a more percussive attack, almost like playing with a pick. One reason I believe my calluses disappeared is that, IIRC, I played stainless steel rounds back then, primarily Rotosounds. Now I play either nickel rounds or flats. So, I'm looking for a low tension (loose) round wound string with harsh windings that i can load onto a practice bass, one that I will use for building my calluses. So, loose (low in tension) and harsh - any recommendations?