The "best" way it can be argued is to use an O scope on the outputs of the amplifier and adjust the "gain" until you see the wave form begin to clip. Since most people do not have access to one of these, there is another method that provides good results for a fraction of the expense and expertise. This is only "easy" if the manufacturer provides the information to the consumer; JL Audio is the only manufacturer that I know of that provides all the voltage numbers in the owners manual. Here's a Flash tutorial that illustrates the process: http://mobile.jlaudio.com/support_pages.php?page_id=143
You could download a 0dB reference sine wave, burn it to a CD and use Ohm's Law to calculate the output voltage. Or you can challenge the tech support people at the manufacturer you've chosen and ask them for the output voltage..... be prepared to be put on hold for a while though. JL Audio does sell a tuning disk if you don't want to search the internet for the deal.
As far as bass boost, the best setting is zero. If you find that you need to boost your bass more than 6dB, you're better off getting more output by adding woofers (or replacing them with real ones
). It's better to get output with displacement over clipping. You need to increase output by 3dB to get the minimal increase humans can hear easily. You need to double the average energy to achieve this increase.... physics can be a b*tch.