You're comparing 2 totally different cabinets. While wood is one variable, there are lots more at play here.
The Mackie 450's are a harsh beast for sure. I wish they would all just go away already! The old Peaveys were not horrible speakers as long as you were able to contain them. Once you started to drive them near their limits, they sounded absolutely dreadful. The advantage of that though, is that they sounded really bad before they got to the point of blowing. Other speaker designs sound just fine right up to their limits, then poof, nothing once they blow out.
Anyway, one of the biggest things that can cause hearing fatigue is distortion. You can comfortably listen to clean audio at a pretty high volume. However, listening to something severely distorted even at relatively low volumes can cause fatigue pretty quickly.
Example, take a crappy boom box and put it in a large room. Crank it up until it distorts to mush. Try and have a conversation next to someone, and you'll realize that it's nearly impossible! If you measure the level, it might only be 80db, but it causes problems with conversation. On the flip side, you can have a clean system pushing over 100db and can have a conversation with someone next to you just fine. It's pretty fascinating.