With metal cones what is usually seen is distortion amplification due to the cones resonance at exact divisions in the harmonic distortion measurements at those divisions below the fundamental. In other words lets say the cone resonates at 9kHz, if you feed a 3kHz signal to the driver its motor will generate a 3rd harmonic at 9kHz, because the cone also rings at this frequency the cone resonance will amplify the distortion product. Here though we cannot see any dramatic evidence of this. Yes there are small peaks in the 3rd and 5th harmonics but nothing that would cause you to dismiss the driver.
Indeed the performance is really very good. The only real fly in the ointment is the increase in tall order harmonics between 300 and 700Hz. This is something to watch out for because it was caused by the driver's frame resonating against the wooden enclosure. The W1-2121S does not have any of the usual methods for mounting. As can be seen from Tang Band's own pictures the rubber ring that secures the half roll surround protrudes every so slightly from the moulded plastic basket. I figured that the best way to mount the driver would be to use this rubber ring to seal the driver into the cabinet from behind. This was a good idea but for a perfect fit the driver needs to have pressure applied from behind to prevent it from vibrating against the cabinet at certain frequencies. This isn't the end of the world but if you want to mount the driver for the best perforomance this is what you need to do.
The degree to which this problem manifests is related entirely to how hard you push the driver. In the above measurement I performed it at 2V RMS. As the driver is 4 ohms this results in about 79dB from the driver, this is quite loud. Most of the time I actually listen to them at a much lower volume and at about 30-40cm distance away, so felt a 1 watt drive level was fairly appropriate given their end use.
Given the drivers inherent linearity the only thing necessary, from a crossover point of view, was to compensate for baffle step with the tuning of the passive radiators the only thing left. As bought from aliexpress they came with no specifications so were a completely unknown quantity.