Energy C5 or C500

How much specifically are the 5's? You can get C-500s from Audio Advisor and elsewhere for around $400/pair.

I heard the C-5s long ago, but haven't heard the C-500s. Many of the specs of the two models are very similar--frequency range, driver complement and sizes, cabinet dimensions and weight, and sensitivity. The biggest difference I noticed is that the C-5 has a single large diameter port on the front while the C-500 has two smaller dia. ports on the rear. That would change the room placement flexibility, the amount of rear wall bass reinforcement, and--depending on how those twin ports are tuned--the loading and damping.

Energy claims more dynamics, speed, and slam over the old model. Evolution in driver design and performance could easily account for this.

I'm not sure why you're saying the C-5's are reputedly better. In my experience, API speakers don't change models unless they display significant improvements over the predecessors. I own four generations of Mirage speakers that reinforce my perception.

The 500s are not significantly lighter. Here are the specs for the C-5. They weigh 47 lbs. each. Here are the specs for the C-500. They weigh 44.1 lbs. each. Just the use of a lighter, more efficient magnet could account for the difference and would signify an improvement. Also, a more sophisticated form of internal damping could account for lower weight. Other dimensions are usually within an inch of the old model. The old C-5 used injection-molded polypropylene mid/woofers. The new model uses composite Fiberglas cones, which is probably a step up. Mirage's (same parent company) flagship speaker ($7500/pr OMD-28) is using a form of Fiberglas for its woofer cones as well.

Every time anything changes models, there's always a chorus to whine that the old model was better. It's not necessarily so. In my experience with the later generation API speakers, they have a faster, more articulate bass with more dynamics than the previous models did.