Can you elaborate on the semi-Tia bass drivers? The lows/mid BAs aren't regular BAs?
They are, but they're not implemented in the standard way. A standard BA arrangement would have the driver's spout fire into a sound tube. In the A18, there are two clusters of eight drivers, for the mids and the bass respectively. These groups of eight each fire into a scaffolding (the funnel-like structure visible in transparent A18's) and then into the single Tia bore. As a result, you're somewhere between a typical BA setup and a Tia BA setup, hence the name semi-Tia drivers. The high-mid driver is the only one with a tube attached to its spout within the whole IEM.