Besides being much handier to change strings through the top load slot, I believe you get less wear and tear on your strings. As you pass a string through the hole in back end of your bridge you cannot help but rub the string winding against the edge of the bridge. If the string is rubbed hard enough, you could deform one side causing damage (similar to the way you would rub scissors against a ribbon to curl it). A deformed string may not orbit correctly, causing false overtones. Also you avoid scratching the face of your instrument due dragging the string ball against the finish.
Yes, all of our A style bass bridges are string thru, please specify when ordering vintage or B style bass bridges so we can accommodate your requirements.
Aluminum bridges are lighter in weight so if you are concerned about your instruments over all weight than aluminum is the way to go. Aluminum tends to deliver more mids and highs while brass accentuates the low end tones.
Here is what you need to do. Measure from the center of your highest string to the center of your lowest string. Lets say it is 3.75 inches on a 6 string. You will need to divide this number by the number of spaces between each string. In this case there are 5 spaces. So if the string spread is 3.75, we divide that by 5 and we end up with a string to string spacing of .750.