Good powered subs do bass best. Whether its needed or not depends on all the rest.
Lots of good subs for use with monitors and other speakers with limited bass extension available for under $1000.
If speakers are more full range to start with, then you need a bigger sub that is also good quality and can cover from your crossover point down to 20hz or so, in order to be considered "full range" in regards to what most people can clearly hear. There are fewer to choose from here for under $1000, but still many good choices I think.
Where to crossover will depend case by case on speakers ability to cover higher bass frequencies versus the subs. A good sub that can do it all well from 20 hz up past the range that most good speakers can cover well will be rarer.
And yes, adding a sub and lessening the bass demand for the amps driving the mains is pretty much always a good thing and benefits everything, not just the bass. If done right of course.
If you can find two good subs to fit the bill within budget, then two subs are always better than one for delivering balanced bass response throughout the room.
And 4 are better than two for that.
It never ends for an audiophile, but probably significant diminishing returns after that.
