Originally Posted by Arjank
Though doubling drivers only increases SPL with 3dB, same as doubling the amp output
This is a good example of my oft posted 'it depends'. +1
If you start out with a relatively low powered amp, and a small cab (like a 210) that has relatively good mechanical specs, it is amazing what a 2db increase in output, and the additional headroom that allow a more open, non-compressed low end can do. It can literally be the difference between 'this rig sucks' and 'this rig is great'.
Start out with 500 watts into 8ohms and a 210, you would get zero advantage of going to a 4ohm unit, and would be much better off with additional drivers.
It all depends