I have been using hearing aids for birding for about 8 years. I have 
external digital aids with "full" earplugs. I have only high frequency 
loss with the loss starting at about 3000cps and dropping rapidly.  I 
tried some of the high frequency-only aids and would suggest they work 
fairly well. The problem with any aid is trying to maintain "natural" 
sound sensation -- I finally decided to forego "natrual" for no 
feedback. To get natural, you use open ear plugs which then allow for 
lots of feedback (screeching from the aids). The high frequency-only 
aids do not have this option. Especially with a hat or other object near 
the aids, you get feedback if they are set to high-enough levels to be 
useful.

I suspect the better approach for high frequency loss are the 
frequency-shifting "aids" The "Song finder" (http://www.nselec.com/) is 
far cheaper than most hearing aids (it is still expensive!) and works 
very well for this type of loss. Ultimately, shifting the sounds to a 
frequency where your hearing is good is better than trying to simply 
amplify the high frequencies as it is impossible to amplify them enough 
to reverse the loss in most ears at the higher frequencies. I think 
there are some in/over-the-ear devices that may offer this 
frequency-folding mode, too.

Best,

Joe McDaniel
Garrett County
Grantsville, MD