Subwoofer with good monitors

I have monitors that goes down to 39Hz so I would like to add
Subwoofer just to reinforce the bass. I'm not looking for
Subwoofer that goes to 16Hz or 18Hz. I would be O.K. with one
that goes to 25Hz. Also, I really don't want to use external
crossover. Would REL R305 do the job or Velodyne DD10 so I
can use built in crossover. My room is 17' x 11.5' x 9'.
Thanks in advance on Yours opinions.
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.
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!
Jl Audio Fathom F-110 would be something you should look at.
What part of the country do you live? You should always try to hear the sub in your own system if you have a dealer/manufacturer that will accommodate. I may be able to help.