I hate to be the bearer of bad news, but rental of personal property (such as sound equipment) should not be reported on Schedule E. Schedule E is for rental of real property only (or, in some circumstances, personal property that is rented along with real property). So, while the production company accountant may be being stubborn, technically it makes no difference which box he reports your rental income in--that income can only be claimed on Schedule C no matter what box it's reported to you in. And consequently it will be subject to Self-Employment tax.
If you make enough in rental income each year to justify the effort, it may be worth forming a pass-through entity (such as an LLC or S corporation) and rent all your gear through that entity. If you structure things right, after depreciation of your gear and other operating expenses, you might be able to significantly reduce your Self-Employment tax, or eliminate it altogether. Of course, you would have some other taxes and reporting requirements to contend with. But, like I said, if your earnings in gear rental are substantial enough, it might pencil out in your favor. Ask your accountant for his/her advice about creating a pass-through entity.
P.S. I'm not a tax CPA, nor do I play one on television, so please don't consider this professional advice. Just speaking from my own personal experience.