Right now, explosions sound like exploding rubber tyres, giving a distinct "pewwww" sound rather unlike anything I've heard in real life. These sounds are rather disappointing to listen to, considering the general quality of the weapons sounds and vehicles.
Explosions are extremely sharp, -loud- bangs at the top end of the frequency range. The high pitched fractions of the noise get absorbed in the atmosphere, which is why faraway explosions sound appears stretched and bassy.
Rather than putting the echo into the sound itself, use a filter. The sound of the explosion going off itself should portray the violence behind it. The closest I´ve come to an actual explosion going off was standing next to a restored 9 pounder artillery piece firing blanks. Your ears ring after that.
A handgrenade going off is about as loud, and sharp. Same for the 40mm grenades. An AT missile impact or vehicle explosion is -much- louder and more violent, and almost exclusively, a single, sharp, sky-splitting bang.