I use the foamies - the silicone can reverse the attenuation of the high frequencies, but at the cost of less isolation. I also like the Comply P-Series foams. I have usually been able to offset the high frequency cutting effect of foams by pushing back toward the IEM when I compress the foam before insertion. I know when I have it correct when I feel the hard plastic of the inner core pressing against my ear canal...
I cut off the tip of the Westone foamies & expose the inner tube , high end opens up a bit (or atleast it seems so)