These are not that great for 35mm. You are probably not scanning at an optimal focus height. That's my experience.
You can adjust focus height with shims, but only up. Or you can shim up from the glass deck.
I suggest you need to accomplish two things... a way to flatten the curvature of the neg.. 35 often has a fairly curved set to it, and a way to adjust height.
I don't really shoot 35mm, so I just passed on the process.
however, I understand that the scan holders from www.betterscanning.com
offer both ANR glass covers to flatten the slide, and adjustment for height.
I don't use anything to post process the image at the scan level, and I DON'T use Ice. I only use the Epson software.
I understand that some people "profess" to get better results with products like VueScan, and others. My experience is that any image process done at the scan level seriously reduces the work I can accomplish in regular Post Processing. In particular sharpening at the scan level, really reduces any further sharpening I can do in Post Processing software.
In addition, I follow the school of thinking that reserves sharpening as the last step in all post processing... certainly not the first.
My goal is a flat, focused negative at 2400. The rest, I have more latitude in any other post processing software.
But start at your worst enemy in the process. The film holders that come with the scanners.