Hey, you started with the same IEM as me, the Denon AH-C551!
I know how you feel. I think I had the exact same feeling as you when I got those. It was 100% better than my Apple earbuds.
I've bought a Ultimate Ears Super.fi 3's a few months after I got the Denon's, and sad to say, I never liked the flat sound signature.(Head-Fi has really hurt my wallet.)
As IEM's became cheaper over the years, I was finally able to purchase the Triple.fi 10's for half the price of what they were selling back then. ($145, for me at least, is a very good deal for a triple driver IEM. Plus, I sold my Super.fi 3's and the currency rate helped it make around $80 in total.)
I've wanted the TF-10's ever since I got on Head-Fi, but the $300 price tag really put me off.
Although I'm satisfied with BUYING the actual IEM, but I just can't appreciate the sound. I've encoded a lot of my music in Apple Lossless, bought the SONY tips to get the exact fit, did the flip mod to help with fit also, but it just doesn't seem WOW as the Denon's had on me before.
I guess you just have to actually take time to listen to them to actually know you like them. I'm thinking about getting 'dynamic' driver IEM's as I think they provide the sound signature I prefer. (IE8, Atrio M5 with MG7 driver, MTPG, MTPC are some, but they're so much expensive compared with the TF-10's