Subwoofer with good monitors

Have you actually done a tone check to verify the response from your speakers? You'll be the most happy with a large driver subwoofer that can really go low, at least in my opinion. I used a ML Dynamo 700 subwoofer for home theater, but generally don't use it for 2 channel listening.

I just had installed a REL R-305 in my 2-channel system that has a pair of SF Guarneri Mementos (go down to 39 Hz) driven by an Accuphase P-3000. The room is 19' x' 13' x 8'. Seamless integration. A dramatic improvement.

It can be very difficult to integrate an aftermarket subwoofer. Timing and phase mean a lot and weaknesses in either work their way into higher frequencies.

I have a pair of DIY sealed box subwoofers that give me in-room response down to 10hz or lower depending on room position / listening position. They use a 4th order active low-pass filter at 40hz and only operate below that frequency. I could not get the phase and timing to integrate with my "main" speakers and like Mceljo, no longer use the subs for typical two-channel music.

I agree that to really understand your needs and to maybe help improve set-up for bass response, you should consider taking some in-room measurements of the speakers. Small changes in speaker placement can change the bass response quite a bit. Some years ago, Allen Perkins wrote a paper on speaker placement that might give you some ideas on bass optimization. Google is your friend.

Generally, despite all claims, monitors have little power response below twice the -3db point. Room lift at certain frequencies adds the illusion of bass and sounds fuller, but a decent sub will add real power response at say 40-100 HZ.
You will immediately notice the difference. Remember, the lowest note on a double bass is 42HZ. Many (most) rooms will behave badly at lower frequencies.
This matters little with HT, but a lot with music. A sealed box sub will not go as low as a ported box, but it may well be easier to match to most rooms.

Vandersteen subwoofers have a unique way of connecting with your system and integrate extremely well. As long as you are using a separate amp and preamp the Vandersteen 2W or 2Wq would be my first choice for a music only system.