which speaker to buy

Just because he can bridge doesn't mean he should.  For efficient speakers like Klipsch, he'll only be using a few watts at most before driving them to hearing damaging levels.  It's good to have that extra headroom for dynamics, but bridging them is a dumb idea.  If he goes for Klipsch, some of the more recent (III or IV) Heritage speakers would be the way to go in my opinion.  Klipsch will do loud and will rock out, but they are not the most refined speakers out there and may play loud, but may not sound that great when they're really loud.  I have owned and listened to a lot of Klipsch speakers and almost all of them hurt my ears at loud volumes.  I had a pair of KLF-30s that I really liked but those are hard to find and they have issues with the cabinets ringing and falling apart.

Legacy Audio as mentioned makes more refined speakers than Klipsch that can play very loud.  I had some Focus 20/20s and those were beasts and sounded noticeably better than the KLF-30s at high volumes.  I sold them eventually because they were just overkill for my space.  They were hard to let go.

I was considering some Zu Audio speakers recently but decided against it.  I talked to a number of people about them and the consensus was that they do very well with low watt SET amps but aren't necessarily a good match with higher powered amps (I have 180 WPC tube monoblocks).  I did not hear them and have no direct experience, so take that for what it's worth.  

The Tektons look "interesting".  I'll just leave it at that.  

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