I had the same problem; what worked for me was a combination of heavier strings (D'addario J75s or 74s) plus a heavy Wegen pick (at least 1.5mm) plus paying a lot of attention to the angle of the pick as it strikes the strings. I spent hours experimenting until I felt I'd got the most out of the bass. Re the recommendation for Will Parsons; yes, his mandolins are excellent and very good value, but in my experience his customer service sucks big time. For real bang for the buck, try Eastman mandolins. Good looking, affordable and some of them are real tone monsters. Or look for a second hand Rigel A+. One other thing; Red Henry did a lot of research on the effects of using a solid maple bridge incorporating the sort of designs that violin bridges use. I thing the results are available in the archives at the mandolin cafe.
Tim