Just looked at the THB6064 datasheet and it's certainly a big improvement on the TB6560. It claims 50V and 4.5A, which if true would be a good cheap option for driving the common 3Nm stepper motors. Although those are the absolute maximum ratings so might get away with 4.2A but the voltage will have to be less than 50V...says in the datasheed 4A and 42V which is not too bad. It's about £16.50 for two ICs on eBay, so quite a significant saving over an m542 should be possible. Maybe I'll get one for testing purposes.