Complex sentence: National countered the Dobro with its own single resonator model, which had previously been designed by Dopyera before he left the company; while also continuing to produce the tricone design which many players preferred for its tone.
Split into simpler sentences: National countered the Dobro with its own single resonator model, which Dopyera had designed before he left the company. They also contined to produce the tricone design, which many players preferred for its tone.