In current master, if no output channels are specified, ASIO401 never starts the QA401 and never reads anything. The host application will only get silence.

While investigating this I also noticed that the approach of skipping the writes if no output channels are specified doesn't seem to work - if I try, the first read blocks forever. In other words, it looks like the QA401 doesn't support read-only mode (although write-only does work). One implication of this is that writing 5 into register 4 is not the the only thing that's required for the QA401 to start streaming - there needs to be data in the output queue, as well.