Same sentiments when I first got the MEST II... until I got them remoulded to CIEMs and escape the never ending tip rolling...Settle on one and don't swap. The 07s haven't left the IE 900 for months now.
As above, AET 07, 08 or SD were all more comfortable and better sounding than stock for me. Final E are comfortable and attenuate treble, but mess up the mids on almost any IEM I've used them with, including IE 900. Might work for you though. Tips are relatively cheap and easy to try on for size, fit and sound. Well worth the time and money to get it right - too many have given up on the IE 900 too soon by losing patience.
For some reason, here I am, back at it with the IE900 >..< But, I have no regrets. the IE900s are tip dependant, but NOT as dependant as the MEST II were. Waiting for 07s to come in, meanwhile, with the Final E which are the most comfortable with the best seal that I have so far. The Xelastec seals well, but I never liked the sticky feeling when removing them, and knowing I can never clean them fully...