No vocals & mid mids

Question, is the vocal part in mids (using splits) dependant on the splits used and AMP or mostly the actual speakers?? A friend has changed AMPs yesterday, from a 1000W peak 4 channel (bridged to 2) to a 1200W peak 2 channel AMP and claims that the vocals on his splits have disappeared (he has plenty more highs (tweeters are louder) and some lows but 'voices' have gone down significantly). He triple checked polarity etc and check out ok. He is somewhat overpowering the splits but above is experienced at low and high volume. The setup sounds little better with treble turned down all the way and bass up all the way.

In your view, could components loose mid mids when AMP either little stronger or weaker for that matter??

Both AMPs are set to full range. The splits (Calibra Stealth) have passive crossover gadget.. With AMP set to high pass,  the problem worsens..

The 4ch AMP is a Blaster 1000W (peak) and the 2 ch is a Starsound 1200W (peak). The splits are Calibra Stealth components. The crossover gadget has switch for tweeters i.e. 0db, -2db and -4db. He has set switch to -4db but tweeter still very loud.

Yes there is a sub but that runs from another AMP (RF 502S). The sub was off while we struggled with above. He wants to put the 4ch back. I just find it hard to understand why above is happening and want to learn for future... Could it be backwaves that cancels vocals? I guess there is nothing I can do but recommend another set of components or put his old AMP back