Good question but tough to answer without knowing your desired SPL. As an extreme example, even a 10 WPC amplifier could have enough "power to drive your speakers if your desired max. SPL is, say 70 dB at a distance of 8 ft from the speakers.
You cannot go by the volume position at all, though in most cases, if you never turn it up pass 1/4 then you are likely okay.
For a more meaningful assessment, please try the calculator linked below and see what it tells you.
Peak SPL Calculator (homestead.com)
The calculator's sensitivity field is based on 1 W/1 M, whereas the Elac's sensitivity spec is 87 dB/2.83 V/1 M so when you enter it into the calculator, don't use 87 dB but 85.7 dB for better accuracy.
Also, it is better to follow THX standard, that is based on 1 speaker. If you post the results then we can try to answer your question.
FYI, reference level is 85 dB with 20 dB peak so max. peak will be 105 dB, again that's for one speaker so it is very loud, similar to that in a good movie cinema.
WHAT IS THE REFERENCE LEVEL? - THX