There are multiple problems possible here. Driver as well as amplifier gain mismatches, not to mention the difficulty in finding drivers you really like which happen to match in output, as well as equalization. Plus, whatever you do on paper you'll probably want to alter somewhat.
Amplifiers do not all have the same gain. There’s a convention for 23 dBV but it’s not a legal requirement and experimental boutique 6W amps may not follow that at all.
In your case, you may be best served by a very efficient tweeter, which maximizes the output from the 6W amp, and appropriate level controls before the amps.