I test various lines all of the time and have found Berkley FireLine Fused Original to be incredibly strong. I've been tying the Fireline to PLine CXX (6lb test in both products) using Yucatan, Bristol, Stren and Uni to Uni knots getting crazy good results. The results show it will hold out to 17lbs on the top end and 12-14lbs on average...of course these are crude tests in my garage using a 50lb digital scale. The Stren Knot is probably the best knot for marrying small diameter stuff but would probably go to the Bristol or Yucatan on heavier stuff.

I've been comparing it to the Power Pro which is rated at 5lb test and the 6lb PLine CXX which is a mono and Fireline Fused Original outperform the Power Pro every time. The PowerPro when married to the PLine usually succumbs at about 10lbs of force.