Vish,
I don't think it's possible to find a true bookshelf monitor that will go down to 20hz. I also don't think it's really necessary. I believe that the lowest note an electric bass can hit is 30hz, and a speaker that goes down into the upper 30's would be more than adequate unless your daughter is a bass junkie. I use a pair of monitors, JMlab Micro-Utopia that are rated to 50hz, but in room get down to about 40hz, perhaps even a bit lower, and I'm completely satified with the way music sounds, even bass heavy music. I'm no engineer, but given the small size of your room, you may not even be able to support bass notes as low as 20hz. If you truly want low bass a sub will be needed, though again in that room a good sub-free monitor may really be all that you need. At least you could start with a monitor and add a sub if needed. There are a ton of great monitors out there, in your price range perhaps something from Paradigm, PSB, or Totem might suit. I'm sure others will have other suggestions, good luck and happy listening!
I don't think it's possible to find a true bookshelf monitor that will go down to 20hz. I also don't think it's really necessary. I believe that the lowest note an electric bass can hit is 30hz, and a speaker that goes down into the upper 30's would be more than adequate unless your daughter is a bass junkie. I use a pair of monitors, JMlab Micro-Utopia that are rated to 50hz, but in room get down to about 40hz, perhaps even a bit lower, and I'm completely satified with the way music sounds, even bass heavy music. I'm no engineer, but given the small size of your room, you may not even be able to support bass notes as low as 20hz. If you truly want low bass a sub will be needed, though again in that room a good sub-free monitor may really be all that you need. At least you could start with a monitor and add a sub if needed. There are a ton of great monitors out there, in your price range perhaps something from Paradigm, PSB, or Totem might suit. I'm sure others will have other suggestions, good luck and happy listening!