If you're sitting in the front row of a concert, classical music can range from 75dBC to 110dBC (headroom of 35dB). I estimated these figures from a study that was conducted in the UK.
You can use this
calculator to figure out the power that you need.
I entered a distance of 2m, a desired level of 75dB, M3 sensitivity of 96dB (because you will have two of them) and headroom of 35dB. This calculator says that you need 100 Watts.
If your room is "quiet" and your hearing is in good order, 100 Watts should be sufficient. You may be able to get away with less depending on your room acoustics.
