Here are just a few of the studies that have looked at the effects of hearing loss and hearing amplification.

Long-Term Effects of Monaural, Binaural and No Amplification in Subjects with Bilateral Hearing Loss
Scandinavian Audiology, 1987, Vol. 16, No. 4 : Pages 201-207
Stanley A. Gelfand, Shlomo Silman, and Leslie Ross

Adult-onset auditory deprivation.
J Am Acad Audiol. 1992 Nov;3(6):390-6
Silman S, Silverman CA, Emmer MB, Gelfand SA

Hearing Loss
National Academy on An Aging Society, 1999, 2, 1-6