You may also be running a load thats too low for the Concert Bass.
I'm not a big transistor Sunn expert, but at the Link Removed there has been lots of discussion over the years about Concert Series amps and two ohm loads. The general consensus among forum members who are transistor Sunn savvy is that it's a bad idea to run one at 2 ohms; even though later model Concerts were rated at 2 ohms.
If you dig around on the Link Removed you will find several threads on the subject, but I think this one sums up the reasoning behind NOT running a Concert at 2 ohms pretty tidily;
Link Removed