My second eyebrow has now joined my first, far north of my eyeballs hearing you have four subs. How did you know you needed more than two?
A lot of guys are going the four sub route including myself. I started with one, then two and now four. Not to get more bass, but to get better bass. Multiple subs are more of an electronic version of low frequency room treatment. They greatly help in smoothing out the low end frequency response of a room. There are a great many threads on audiogon that discuss the merits of multiple subs, aka DBA, distributed bass array. Also see AudioKinesis "Swarm"
How were you able to dial-in all four?
Personally, I found it much easier to dial in four than one, because four reduced or eliminated many of the problems usually associated with subs, but I did recently fine tune my set-up with the help of REW.
Next time, if there's a next time, I push past 60% I will have the tone control disabled and will be watching the power guard lights with my finger on the mute or volume control.
Not knowing what track you were playing when the incident happened I strongly suspect it was the 25 Hz control that caused the problem. You might try setting the 25 control at neutral or lower, increasing the 50 to 3/4 and reducing the 100 to 1/4 and see if that sounds OK to you, though you still might not get much more volume. In my opinion a 7" driver is just too small to reproduce loud bass much below 60 Hz.