Hello to all.
I have a pair of Roland DS-7 monitor speakers and I was wondering if I should get a subwoofer to get those low frequencies.
I'm using Sonar professional and trying to produce electronic music.
Thank you.

you know,
maybe in your room, those speakers are fine.
 
but if you need more depth to feel the mix in the low end,
the answer is yes.
 
which one to get, oh, that's the mind phook

 
If you use appropriate reference material and learn your room and monitors, you may not need the sub.  You might also find that your room isn't compatible with the sub and/or requires massive amounts of treatment to manage the low end.
 
Actually, whether or not you get the sub, you will have to learn your room and monitoring system.
 
 

 
What size is your room ?  Those monitors shoud be good for at least 15' x 20' x 10'.
 
I wouldn't worry about a subwoofer unless you're producing really bass-heavy* music and have no neighbours.
 
[* depends on what you mean by "electronic music"]

I would get a sub... although I agree with a lot of the above, I feel a sub really lets me taylor/sculpt the low end better... if a signal is there, you will hear it and if you don't want it, you will be able to eq accordingly.
 
 

One small recommendation...
 
If you get a sub, get a SMALL one, no more than a 10"driver. I got a 12", and have regretted it ever since. It overwhelms my room.
 
Live and learn.

It is usually best to keep things as simple as possible.  In a med room a 7-inch woofer should suffice for most music.  If the lack of bass and volume are a problem, then yes.  The smaller Yamaha (10-inch I think) used to be a reasonable deal.

Speakers throw bass forwards AND backwards. The rear part hits the wall, bounces back and reinforces or cancels and messes with what you hear at the low end.
 
For this reason, manufacturers say your speakers should be 4ft or more from a wall. If you get a sub, it removes the lower bass from your speakers and reduces this interference. The sub takes it over but the sub is designed to be on the floor.
 
One important thing: the sub should be just loud enough that you hear no difference in overall volume. You should just be aware of some more lower end. Ideally your subs should have a bypass switch but these tend to be only available on the more expensive ones, the best even come with a remote. Without one, you will have to bend under your table to unplug and replug to compare. Tiresome.
 
So I built myself a box with two bypass switches so I can A/B whilst at my sitting position.
 
Fluid Audio have released their F8S. Footswitch bypass! And a continuous phase control! Wish I'd waited.

jpetersen
Speakers throw bass forwards AND backwards. The rear part hits the wall, bounces back and reinforces or cancels and messes with what you hear at the low end.

Low frequencies ooze more than bounce.  A 60hz wave is 19' (6m) long, 40hz is 28' (9m) so it's kind of hard for it to cancel itself out (except for a room mode).  It's why you can stick a woofer just about anywhere, the floor just being practical, though it's not the best.  Floyd Toole's book "Sound Reproduction: The Acoustics and Psychoacoustics of Loudspeakers and Rooms" has some great info on subwoofers (more than one being better) and everything else about rooms and speaker placement.  I can't believe this book is freely available on google, but it is (I bought it years ago).  See pages 218-221.

Regarding the distance I was quoting my Yamaha HS8 manual.
 
For the rest, let Andrew Goldberg from Neumann explain.
Start at 2:15 and listen through to 4:50.
https://www.youtube.com/watch?v=wcUiObrrOp4