Hey Sergio, I understand your pain.

Well, the most affordable option is to just supplement with some inexpensive subs. The typical cheap subs (the JBL LSR310s come to mind at something like $420 US each, depending on sales) go down to 27hz according to JBL specs, but the crossover is of unknown quality. Designed to work with powered monitors that often sell for $200 US each.

There are a range of home theater style subs that will let you take a speaker level feed at your monitors and run that into the subs. You have to dial in your crossover point and level. Sometimes just a little bit of extra extension is what you want. I was looking for more radical results, and I got them, but at a price!!!

For your purposes, how affordable do they need to be?