I have $300 pair of Mackie MR-5's ... 5" cones don't move much air for the bass. So I bought a 10" 100w powered Polk Audio sub when I saw it at a good price. ( on sale for $250 IIRC)
Best thing I ever did for the sound in the studio. Well, maybe not the best, but definitely up there in the top 5.
Once you get it set up and adjusted properly, you can mix the lows because you can now hear and FEEL them. Electronic music and synth stuff tends to use lots of really low synths, so you want to be able to hear that stuff accurately and the $300 or even the $500-600 monitors are not going to be able to deal with that program material effectively.
Even the bass and kick drum in country music (which is what I tend to mix most) is really easy to hear with a sub. Been to a country concert recently? The bass and kick are in your face. Not much wimpy low end these days at country events.
Yeah .... buy a sub. For a home studio, find one on sale and buy it. IMHO... a home audio sub will do fine. That's what I have.