Earplugs
 

I hear wearing earplugs under your heatset helps preserve your hearing, at the same time you need to turn up the volume even more.

Anyone knowledgeble?

It makes sense to me. That's how I work out at the gym (fact: earbuds lead to hearing loss.) From day one I've either worn David Clarks in pistons or earplugs under lighter Telexes in jets.

If you're really serious about hearing conservation, these are what you want. Basically ear buds that you wire into your headset/helmet surrounded by ear plug foam that you then wear under your headset/helmet. If you're really serious, you can get custom molded ones.

Communication Earplug Kit

They work really well and allow you to have the dual-layer protection from ambient noise (plugs and cups) and still let you hear the ICS/Radio/Warnings. If you're doing long hours in a SE piston or turboprop, I'd say it's highly recommended, probably overkill if you're in a big multi turbofan.

I use the CEP system with my flight helmet (HGU-55), but not with headsets. I also use Oregon Aero kits for liner, earseals, etc, for improved noise attenuation.

I use Bose headsets, usually no ear plugs. In exceptionally loud aircraft where a headset is worn, I may use passive headsets with Oregon Aero hush kits (extra foam, better noise attenuation), and ear plugs beneath. There does come a point of diminishing returns when one needs to turn the volume up loud.

For earplugs, I carry Radians custom molded plugs. You mold them yourself. In lieu of those! The foam expandable in-ear plugs are hard to beat.