440 Source Aluminum Big Block Heads

Just my experience, but it's rare for a valve to break without some form of input... Meaning a retainer/lock failure, the impacts like IQ said, bad valvetrain harmonics, poor valve jobs, etc. Most of the imported stainless performance valves are ok parts, and virtually everyone uses them. If you replaced the springs, that's good. If you didn't replace the retainers and locks, or have the valve job corrected, you may have missed a potential problem.
Good head work is hard to find, and more pricey.