Adjusting latency in fine increments

You should be able able to type in a specific value for the LatencyMSec value in AUD.INI. This value is in milliseconds rather than samples, so you'll need to convert your desired buffer size from samples to time. At 44.1 KHz, a buffer size of 256 translates into a latency of 5.8 milliseconds. 128 samples is half that, or 2.9 ms.
 
However, I can't imagine why a device would require a specific buffer size. More likely, the vendor's recommendation is just that: a recommendation.