Nothing is going to compare to the sound of IEMs. Nothing. So nothing that I say here will compare sound-wise with that.
However... I've taken to use certain earplugs as being the only one trying to use IEMs in my main two bands sometimes results on me looking like a diva, and on other bands I sometimes play it's just not practical.
I'll still keep talking about IEMs, but meanwhile when I don't, I found finally some earplugs that I'm very happy with. And I'm very surprised at that.
In second place... I got some custom moulded ones made by ACS in the UK. They cost me something like £120 after some promotion, they're a little more usually. I went for their 17db filters which were the flattest response of the set, incidentally.
They work well. I don't *love* them but they work well. They allow enough sound in to not feel too isolated and feel reasonably natural. They sound better than the million and a half other earplugs I've tried over the years.
In first place... a £25 set of earplugs called Isolate. The ones that are just a block of metal and a foam plug. Now, I was very sceptical. They made no sense to me. But a few people I know used them and liked them. I thought they were a little ignorant. One of my guitarists used them too... and he also liked them. And one day, against what my brain was saying, I bought a pair as I figured they'd at least be useful when we go on minitours and have to share a room with the heavily snorer guitarist.
Well, they cut a LOT more sound than any others. It's a bit confusing at first, but you get used to it quickly. For some reason, you still get a nice amount of treble in, and it seems that maybe they block low rumbling lows or something... I don't know what it is, but I can hear everything pretty clearly, and my bass as well, it sounds tight as if I used some kind of HPF onstage. I used to play a lot in this venue with a very boomy stage, where we ended up sounding way too loud for it and terrible terrible sound. Now I am very happy playing there.
I know, you'll read the blurb and say "what a lot of garbage". I did too. But they're cheap enough to give them a try.
I prefer IEMs, of course... but these are second best for me. I just show up and loud volume is never a problem anymore, I can hear everyone and myself clearly in any stage... They always travel with me. They've literally saved a gig for me, where I was ready to quit because of the stupid volume levels that made monitoring really difficult for me.