I haven't listened to X10 for quite sometime now. But if my memory serves me right, you can skip the X10 all together and go for R50 instead. You won't find as much bass on R-50, but it will offer much better clarity and detail. There are definitely better sounding IEM than MH1, but the question is how much you are prepared to spend.
Going from the MH1C to the R50 would be a big shock in terms of sound signature. So much so that I bet he wouldn't like the R50s because they will sound thin in comparison. I haven't heard a ton of IEMs, but for me personally, the Yamaha EPH100 was the first true sound upgrade to the MH1C that retains a similar, more musical signature, even though i love the R-50s. Clie, have you tried the TDK BA200s? According to others, they seem like a good option for those that are looking for more hi-fi sound yet better bass response/warmer tone than say the R-50s.