Stik, I have the same problem! My S2A has a Sigtronics SPA-400 TSO intercom and a Bendix King KY 97A TSO radio. So whatever you do, don't get that combo!
The only headset solution I've found that works in it is the Clarity Aloft, simply because it has a generous built in volume control. I've also tried the David Clark H10-56HXL ANR helicopter headset, which is not great even with the headset volumes right up, and a helmet with CEP which is okay but not as good as the Clarity Aloft and much more expensive.