I used to have a 17-79 and while it seemed to work alongside others, it occassionally emitted a high pitched whistle.
Also, its noise reduction was poor. I sent it back to the mfg in the USA and they said there is nothing wrong with it! They even sent me graphs which showed exactly what I suspected! Eventually, after about 30 emails and faxes, I managed to return it to the aviation ship (in the USA too) where I bought it, for a refund.
A lot of the adverts for this headset, notably certain ones by a well known pilot shop, quote misleading attenuation figures (about 20db too high).
I think it's a pile of junk.
I use a Bose X but, 5 years later, there must be better stuff on the market by now, at the same price.
Either yours is faulty (tried it in another plane?) or the intercom is duff. Most spamcan intercoms are primitive designs which don't have a separate amplifier for each headset output.