Isotopic substitution and defect doping have been used to alter the optical properties of two-level systems (TLS) in the two amorphous phases of a-H₂O ice. A large TLS optical density of states is found for a-H₂O and a-HOD in the high-density amorphous phase while a thirty times smaller value occurs for both in the low-density amorphous one. Our quantitative spectroscopic and thermal study bring out a number of new features, among them that LiCl doping of low-density amorphous ice increases its TLS optical density of states, its glass transition temperature and its fragility.