The reason Jason isn't responding anymore is because their business venture ended, per California Secretary of State.
You might go to VineLink and check the jails for Cameron. Otherwise, you may decide he cheapest option is finding him, in person. A good private investigator can track him down, I would bet, for less than $500. If you have thousands of dollars worth of an amp missing and he's supposedly got it, might not be a bad investment.
That or file a small claims court lawsuit, MC likely won't answer or respond to, get the abstract judgment, go to the sheriff's office, have the sheriff get your amp back and pick it up there. But the issue there is finding Mark to get him served. So, the PI still seems like the best option if nobody else can get you in contact with him.