Is there such a speaker...(loud but smooth)?

The entire family from ATC.

Find a pair of Dick Sequerra's Metronome 7.7's: (http://www.sequerra.com/electronics/data/index.htm)(http://store.acousticsounds.com/browse_detail.cfm?Title_ID=36370) These small monitors sound good at low inputs yet will remain very linear/undistorted at high volume levels(wide dynamic range). Worth the search whether you shop new or used. Look(used)for the Mk4's or 5's and you can't go wrong.

ATC sounds interesting.

They have had a following in some rock circles and a few high end night clubs/concert halls. So go ATC if you want very loud but Hi-Fi rather than PA sound quality. PA horn type speakers will still go louder more cheaply and be even more indestructable...

It's been a long time but I remember that my good old JBL 4311b control monitors could play really loud and still sound good, not concert levels though. Really miss those speakers. There are probably vintage and current JBL (pro market not best buy stuff) speakers that would work for you. Tyler Acoustics has a line of efficient horn speakers.

The big Tannoys with 15" dual concentric drivers, e.g., Westminster or Churchill.

Also, the Tannoy 215 DMT II professional studio monitor, if you can find one on the used market.