What s the minimum power .....

Jair, a lot depends on what sperkers you are using! That information and the dB efficiency would be extreamly useful?

Jair- Lak is quite right. The efficiency and the impedance (not just the nominal but also the min/max) of the speaker would be needed to further assist in a recommendation. Also, keep in mind that not all "watts" are created equal. I have 50wpc amps that dance circles around 200wpc units in terms of breathing life into the music. Your amp(s) price range would also be helpful in suggesting a solution. Good luck.

You will also need to control the room resonances. If you try for row ten volumes with full orchestral SPL the room modes will overwehlm your listening experience. While the previous posters are right that it would take more info to be completely accurate, a rough guess is that with speakers of about a sensitivity of 92 db, with room treatment, a 150 watt amp would suffice and still have enough head room to prevent clipping on peaks. On the best orchestral CD's peaks can be 10-15 db above average spl. orchestra