Also, from the THX FAQ: "Experience Studio Clarity: THX Certified Receivers reproduce studio Reference Level, 85dB SPL with 20dB of headroom."
This can be interpreted as 85dB continuous and 105dB peak, meaning you ideally want an amp able to reach those spikes.
This calculator follows the concept of loudness going down 6dB when range is doubled, and up 3dB when watt is doubled.
Enter sensitivity of speaker and range to calculate watts needed from amp:
Speaker dB per 1 watt at 1 meter:
Note:Watt is a volt/current relationship. More current than voltage may be required for that 1 watt.
Different amount of voltage may be required for different kinds of speakers.
Meaning that in general, you'll want a high quality amp that has a high quality power supply and electrical components.
Listening range from the speaker in meters:
Alternatively, enter your desired dB instead of using THX peak value (105):