Extremely open headphones for deaf dad


my Father is deaf and is training to be an EMT. however because of his 100+ db loss, there are no amplified stethoscopes he can use and hear with out his hearing-aids in addition so i came up with an idea to use his hearing-aids and a nice set of headphones and it worked! only problem is it took a while to get his hearing aids to stop giving feedback.

i have looked and can't find a open air backed headphone so the headphone doesn't: A damage his $6,000 hearing aids ; B make so much feedback ; C is cost i'm looking at a $200 max budget because the stethoscope it's self is extremely expensive  

Maybe the Audio Technica ATH-AD700 Open Air Dynamic Audiophile Headphones :  http://www.amazon.com/Technica-ATH-AD700-Open-air-Audiophile-Headphones/dp/B000CMS0XU


possibly however im looking for very open because of the feedback     

they don't make anything (to my knowledge) that could be considered anything near audiophile grade, but you could look into a set of bone conduction "headphones", people with hearing loss say good things as long as his (have no idea how to spell this) choclia is in tact

audiophile no, he needs to be able to hear faint heartbeats. he could hear more on a better set of cans so id stick with quality