I received my pair of Kef M100s as well, and so far do not share your enthusiasm for the sound. I am getting a boosted mid bass, not much deep bass, and uneven mid-range. Break in, and finding the best eartips will hopefully improve them for me but not the classic Kef Sound I was hoping for yet. Definitely worth the investment and giving them time.
I also got a pair of the M400 on ears
(a couple of colors are still on sale for $80 Kef via NewEgg) These impress me a good bit more. The same warm musical but detailed sound I remember from my M500s. They are also more attractive in person, and look quite a bit better built, then they do in the photos. Nearly everything visible is aluminum or high quality faux-leather. A midrange that really draws you in. Not super comfortable on my large head...but I am working on adjusting those as well.
For cheap but excellent sounding single driver dynamic IEMs that are a deal (I bought more of them than I need this season) the ones I liked best are: B&O H3s, which are
generally available new on eBay for about $45 because of the LG distributed ones being available cheap. And the really excellent Marshall Modes ,
currently $23 on Amazon through 3rd Party Prime dealers, also on eBay. You are taking a chance because there are a lot of grey market Marshall phones, but if you use Prime they are easy to return for nothing if not genuine. The set I got looks and sounds great.