I have done a lot of recording and mixing in my time. I think you have some misconceptions. You are setting up a close mix situation, that will tell you the detail, however the MOST IMPORTANT mix is the far mix. To get an accurate balance for modern conditions, you really do need an FR down to 30 Hz at least. So I think you do need a sub or larger speakers.
In addition, I suspect that you room may well be over treated, many are, and that will not give you a good final product. If you sit close to the speakers in a dead room, you might as well use headphones. As with all speakers, no matter what you are doing, the key is flat mid band response and an off axis response that accurately mirrors the on axis response.
This drive to dead rooms has come from BAD speakers, that have a poor off axis response.