Will a self-directed 401k, self-directed IRA, or self-directed DB Plan generate taxable income if it invests in a business that sells goods or services?

It depends on whether that business is treated as a C-corporation for tax purposes. If that entity is a C-corp, the dividends paid to the Checkbook Control Retirement Plan will not be taxable. Otherwise, the pass-through income will represent UBTI to the retirement plan.