15" speaker or something like Eminence vs Celestion vs EV...

Personally, I'd forget the idea of modding the Peavey and buying a speaker all together and just buy a proper cab to to with the Ampeg.

 

What are you trying to achieve that the Peavey isn't already doing for you?

 

The challenge is, because cabinets are designed around the speaker, a replacement driver isn't going to be as good a match for the cab as the original. If it were it would probably be sufficiently close to the original in sound that there was little point in changing. So instead of a mid price driver used to best advantage, you could end up with a high price driver compromised by the cab.

Given a substantial investment in test equipment you could probably mod the Peavey cab to better suit another driver, but unless you want to get into speaker cab design for the fun of it you might think there are better things to do with your time.

I'd be inclined to get a pair of nice cabs, maybe one now, one later when cash permits, and sell the Peavey as a going concern to help finance the second.

 

The Kappa 15 is a fine speaker but it will NOT make your amp lighter - it weighs 8kg by itself ! The resale value on Peavey gear seems to be a global problem - we know what you mean. It's probably better to be bitten on the resale than to spend more money on an amp that is a dead end for you. The Kappa 15 is efficient and might do fairly well in that box (especially at lower power), but for the long term it's money spent on a very imperfect strategy.

 

I believe you will find that much of the weight is contained within the box itself

 

One of Peavey's traits is that they are built to be cheap, durable and get the best performance from the low to mid tier speakers they generally use.
Light weight was not even a design goal.
Because they made the effort to optimize the box to get the most from the drivers they used, there is not much to be gained by changing them.
The box itself is heavy.
It is also strong.
Your back will break before it does.

We live in a time when you can get some great deals on older gear because of the weight.
At the same time there are some amazing light weight designs out there, but they aren't cheap.
Either your wallet or your back will end up paying the price.