Additional Study Information: The purpose of this study is assess the safety, safest dose, and effectiveness of venetoclax in combination with dexamethasone in subjects with t(11;14) positive relapsed (comes back) or refractory (did not get better) light chain amyloidosis. Furthermore, the purpose of phase 2 part of this study is to compare the effectiveness of venetoclax and dexamethasone versus investigators' choice treatment in this condition. You are being asked to participate in this study because you have t(11;14) light chain amyloidosis that has come back or did not get better with initial treatment. Ventoclax has been approved by the regulatory authorities to treat certain blood cancers but is not approved to treat light chain amyloidosis. Therefore, the use of the venetoclax is investigational (experimental) for the purposes of this study.