The later Concert series should output 50-60 watts into an 8 ohm load, 100 watts into a 4 ohm load, and 200 watts into a 2 ohm load; although far wiser transistor minds than myself question the stability of those amps with a 2 ohm load.
A watt is a watt. Bench your amp and see if it's healthy.
I've only owned one transistor Sunn, so I can't speak for how "loud" they are, but the old tube amps definitely punch above their weight class. The reason the 60, 80, 120, or 150 watt Sunn tube amps got the reputation for being so loud for their rating is because they were rated very conservatively. Those ratings were the original hifi standard Dynaco ratings of minimum continuous between 20hz and 20,000hz, +/- 1 dB, at <.5% THD, uber clean, which is around "3" on an old Sunn tube amp's volume knob.
Crank one up into 5-10% distortion and they are actually putting out almost twice their rated power, and combined with the efficient AlNiCo JBL cabs (you couldn't buy an old Sunn tube amp without at least one of it's matched cabs) making for an extremely LOUD 60, 80, 120, or 150 watt amp - hence the well deserved reputation for Sunn amps being so freekin' loud.
Not sure if the Concerts are any louder than any other similarly rated transistor amp. As I said bench it; a watt is indeed a watt.
I think maybe the post Sundholm era Sunns, including the transistor Concert series, just kinda got grandfathered in under that "Sunn amps are LOUD" banner.
