Additional Study Information: The purpose of this study is to find out if MLN9708 plus dexamethasone improves against systemic light chain amyloidosis better than the physicians choice chemotherapy treatment. Physicians choice chemotherapy treatment is a treatment regimen that is commonly used to treat amyloidosis patients. The study will also determine if MLN9708 plus dexamethasone can lessen the chance that the amyloidosis will cause the heart and/or kidneys to begin to work poorly, cause patients to need care in the hospital or result in death.Patients will be randomly assigned (like flipping a coin) to a treatment schedule, which will be either Group A or Group B, but not both. If patients are assigned to Group A, they will be given MLN9708 plus Dexamethasone. If patients are assigned to Group B, their physician will choose a treatment that they feel is best suited for the patient's disease.