With a great product like Speaker Workshop to test the speaker design why is it important to listen to the prototype? To

1) Verify that the drivers sound the way you want them to

2) Tweak the crossover for best sound  

IMHO driver sound is vastly overrated. You will see people talk about how good specific materials sound. For example, I just read an article by someone who desperately wanted to build two-ways using 5" drivers to get the "immediate attack" unavailable with larger drivers. Pooh. Drivers sound the way their frequency response indicates.

With that said, speaker sound is very dependent on driver selection, port size, and crossover. Changing the crossover by a tiny percentage can change regions of the frequency response that are quite audible. The same is true for port changes. Thus, it is important to listen to the speaker as you design the crossover.

What to listen for

I have found that there are three things I listen for.

1) Balance - do the bass, midrange, and treble sound in balance? This is usually a function of the relative drive of the woofer and tweeter. It is also a function of the porting-induced bass response. Listen to wide range music for this - rock and roll or symphony. I use Celtic music with a harp and deep drums.

2) Even response - are all of the notes at the same intensity when played that way? I tend to use piano music (or keyboards) for this. They have a wide range and roughly equal intensity in some pieces (I usually use Dave Brubek). This points out narrow peaks and valleys in the frequency response.

3) Timbre - do the instruments sound right? This is like balance but a good indicator of the top end of the speaker and the crossover region - where the harmonics live. Cymbals, guitar, harp, and brass instruments are good here (I use brass because my wife is a professional French Horn player so I make her listen).