To my knowledge, there is no literature available on these issue. This could probably be due to two reasons. One is that increasing low-frequency cases high BOOMing effect. In addition, the limitations with transducer may also limit it. One another that I can think of is that increasing low frequency causes increased upward spread of masking.

Due to these reasons they would not have tried. 

I agree with Dr. Vijaya Kumar Narne because of limitations its not used in present hearing aids. The concept of extending low frequencies in hearing aids started in 1970s (Martin & Pickett, 1970; Sweetow, 1977 etc). They suggested that in presence of noise this concept doesn't work. However, the hearing aids studied had narrow range of frequency response. Thus, Sung and Sung (1982) suggested that extending both low and high frequencies significantly improves speech perception even in presence of noise. Beck (1986) suggested that scores improve and there is no upward spread of masking. There are many studies in 70's and 80's which supported extended low-frequency amplification. However, there are criticisms to the conclusions drawn and presently its not used extensively.

Please find below the review articles on the topic which I could find: 

An extensive review of Extended Low-frequency information can be found in the pdf file attached with the answer. 

The important articles related to extended low-frequency amplification are: 

Sung RJ, Sung GS: Low frequency amplification and speech intelligibility in noise. Hear lnstr 1982. 33(1): 20,47

http://www.ncbi.nlm.nih.gov/pubmed/7412226

http://www.ncbi.nlm.nih.gov/pubmed/6870667

http://www.ncbi.nlm.nih.gov/pubmed/7092728

In addition, the only recent study (2000) which I found was a review article explaining the possible advantages of low-frequency amplification supporting extended low frequency amplification. Please find the link. 

http://www.hearingresearch.org/ross/hearing_loss/low_frequency_residual_hearing_revisited.php

Wish this is useful.

Regards,

Prashanth

Thank you sir.....@Vijayakumar Narne and @Prashanth Prabhu P

sir, exteding lower frequencies how does it help patients in day today activities of conversation to the patient ?