Over at the Firearms blog, Top Shot Winner Chris Cheng talks about his Surefire Sonic Defenders and the Howard Leight electronic earmuffs. This is the exact same setup that I have. When shooting indoors, I double up with the EP3s and the HL muffs. It works for me.
After dealing with some other brand of electronic earmuffs, I switched to the Howard Leight's about six months ago. I couldn't be happier. In fact, I just ordered a second pair as a spare and to use when taking other people to the range. I especially like the amplified earmuffs when taking a new person to the range as it helps me hear them talking and vice versa.