T H U N D E R ! D O N E ! D I R T ! C H E A P !
This may be shocking news to those not in the know, but... To get really thundering lows, I have found the following method best above all other current methods (such as EQ, B-strings, sealed cabs/open cabs, string guages, active/passive electronics etc), the least circuitous path, to be positive and if you're series about it, is...
The next time a lightning storm hits your area, watt you do is put your amp outside as high as possible (under a protective plastic cover, you don't want it rained on – duh!) and attach a lightning rod to the ground/earth prong on the mains plug. Best to wear rubber soled shoes and dishwashing gloves while handling the lightning rod, you don't want to get hertz. In fact, go all out and Bring Out the Gimp, that latex suit you keep locked away in M&D's "special" dresser drawer.
If you find this idea revolting, that it's too much on your plate, have any reluctance, or resist,or don't know which prong on the plug is the earth/ground, then you probably don't have a locking "special" drawer, and you should leave the entire procedure to the adult in charge of your care.
Simple, cheap, effective.
I amp not responsible in any capacity for your injuries should you attempt this on your Ohm, so don't be all negative and blow a fuse at me. A grid?