Unfortunately, FHA financing is only for primary residences. If your credit score is over 700, ask your current or other local mortgage broker lender about doing a Fannie Mae DU Refi Plus. It can be used for investment properties and allows up to 95% LTV for Rate and Term Refinancing. See if this applies to you there.
Thus your loan requires placement with a mortgage investor, each with their own underwriting standards and risk limits. The reality of today's more stringent risk based mortgage underwriting will make placement at a rate significantly below your 6.6% a difficult find. However, give it a try! Contact several mortgage brokers and give them your requirements and credit information and they will give you further advice and initiate a search of their investors to try to place your loan. The process is free..........Good Luck!
Have you ever tried FHA refinancing? Perhaps there are some programs in your local area. Look at local lenders and credit unions and see what they say.