Or you could equate it with HP per pound....... a Ferrari FI is more efficient than a F150 because it has less weight for the horsepower than the F150, based on approx 2800# with 500+ HP, vs. 6000# with 175HP from a 302 in the truck(I'm talking about 1990's F150 here).
Speakers are rated for efficiency on how many db they push based response of a microphone to a 1watt signal, 1meter from the cone face. Higher db rating = higher efficiency. IIRC Anything over 94db is usually rated as a high efficiency speaker, and takes less power to reach window shattering levels. 89 to 94 is considered medium efficiency, with below 89db a low efficiency speaker. The lower you go in efficiency, the more the volume knob has to be advanced for a given Sound pressure level. Put a speaker on one channel that is low efficiency (87db) and another of high efficiency (95db) on the other channel. With the Volume knob all the way down and the balance control centered, turn onthe unit and SLOWLY advance the knob so the pointer if facing to 9:00. Note the level imbalance. THAT is the difference between a 87db and a 95db speaker. It takes less power for the high efficiency speaker to hit and maintain a certain SPL than a low efficiency speaker.
The HPM's will handle the 828 with absolutely NO PROBLEM, no matter whether they are the 100W or 200w versions. The 828 will run out of steam before either version.