which speaker to buy

If he already likes the Klipsch sound, then he should listen to a pair of La Scala's. 

If your son wants his music that loud, then you're going in the totally wrong direction.  Home audio hi-fi speakers can play only so loud.   What your son needs is pro sound gear, which is engineered to play at much higher sound levels.  If you're listening to it that loud, fine detail and nuance are totally lost.

To my ear, some of the very most musical pro sound loudspeakers are those sold by Bag End.  Look  for a pro sound dealer at their website.   The amps you currently  have should work fine.  Pro sound gear is typically more efficient than home audio.  But if he really has to have more power, then he should go to the Musician's Friend website and get one of the big pro amps they have there.

All this will cost much, much less than McIntosh and other audiophile oriented gear and do a better job for this application.

I would also suggest Pro Audio Gear.  Designed to "Rock and Roll" large venues.

Suggest NOT Bridging.  Back in my days most pro speakers were 4ohm. Amplifiers that are bridged do not like driving 4 ohms loads.  I would guess there would be 8 ohm speakers available now.  Buy 4 (2 pairs) of speakers.  Drive each speaker with the two amplifiers in regular stereo mode.

Here's one thing your son can try right now to get his spl jollies.  And not spend a cent.  Move his Klipsch speakers into a bathroom or closet.  Both speakers at one end and sit on the other end.  Or each speaker on opposite walls and sit in the middle.  Conveniently, might be the toilet.  He may need it.   CD player straight into the 7300, set the Gains to 11 and Rock'n Roll.

I do not condone listening of any music at high spl levels.  Every rock concert I went to in the 70's, left my ears ringing.  And notice it's effects now.  Might be old age.  But definitely didn't help.

In my opinion pushing 600 watts into home audio speakers is a recipe for disaster, with rare exceptions.   

If loudness is the priority and cost is the issue, bring your Led Zep and your son to Guitar Center and listen to their PA speakers.  

Duke

SVS ULTRA’s I have the piano white bookshelves and I absolutely love them. They list for $1k but will compete with speakers 3 times the price. Ask any owner. I started with SVS Prime bookshelves and they are only $500 for the pair and they are quite good and will completely satisfy 95% of the people out there, for sure. I just purchased a $2550 pair of QUAD Z2s for a great price of $750 used only 4 months and they are beautiful in there own but I only got them because it was such a great deal I could not pass them up. Don’t let anyone tell you to buy one speaker over another unless you can try them out for yourself. Everyone has different sound tastes. I have had a lot of different speakers and the SVS’s have been my favorite overall. Best Buy has them to hear. Check them out, and keep an eye on Craigslist for them in your area. Dogger