My take has for a long time been, that to be loud enough without loss of clean power, the speaker-amplifier should be able to achieve 102dB per channel. Paramount's 8 watts is 9dB re: 1 watt, so speakers of 93dB sensitivity would be called for. This is a conservative number, based on three independent sources, so it's not just my opinion.
1) an average of Stereophile's recommended minimum power for various speakers, over several years
2) The average listening level and headroom used in professional recording, mixing, and mastering studios
3) The THX home theater specifications
However, there are substantial adjustments depending on your situation. The following will reduce the needed efficiency:
1) a small room,
2) a very live room
3) no interest in loud music - i.e. not a symphonic music fan, no need for hard rock at "realistic" levels, etc. (Note: Rock at concert levels need another 10dB at least and will permanently damage your hearing, eventually. End of rant.)
4) A personal preference for music at levels well below what "most audiophiles, most of the time" prefer.
5) Music that is heavily compressed - it has little headroom; pop songs that play on the radio a lot are good examples.