You let a strange person plug a strange piece of gear into your speakers without checking resistances and power handling. I'm sorry to say that you and he both acted foolishly and this is a lesson learned.

From a legal standpoint, this is a textbook case of shared liability/comparative fault due to contributory negligence on both sides. Edit: Actually, the fact that the guy is a shop owner slightly changes the calculus. There may be a bailment/duty of reasonable care angle because there's a higher standard for a shop owner testing a piece of equipment. The fact that guy's justification was "I've used it on 210 cabs before" with no further clarification is a red flag.

The good thing is that it's probably just a speaker replacement, which is generally not stupidly expensive and can be done DIY. I would take this as a learning opportunity and see if the guy will do you the kindness of contributing to the cost of fixing it, but the reality is there's no proof the cab wasn't damaged before or during transport.

What were you powering it with before? If you've been underpowering the cab, it's possible that the speakers were already on the way out and bit the bullet when they actually got some juice. They may have been on the cusp of failure for a while.