Subwoofer with good monitors

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.

Stay away from THX subs for music.They tend to have a peak around 80 hz or so,for more movie impact.SVS,Hsu,have good value subs.There's a lot of good competition nowadays.

FWIW, I recently added an REL B3 to my Aerial Acoustic 5B monitors (60Hz to 22kHz ±2dB, -8dB at 50Hz) powered by a 50w Jolida 302, and found the integration very easy and extraordinarily musical. It's lots of fun and worth exploring IMHO.

Look into the Omega Deep Hemp Sub. This sub was designed as a music only sub to work with the omega line of fast full range single driver speakers. The sub is an 8" hemp driver that reaches down to the mid 20hz range. It will work well with all loudspeakers. I had a pair and wished that I didn't have to sell them. Very,very,very musical!