I've never used Photomatix, so someone with experience in that should likely help. I do my conversion one of two ways. My primary methodology is using Lightroom and exporting a JPG from either the RAW or TIFF file, adding a watermark and resizing in the same step. I am happy with those results and notice no change in color profiling. The other method I use would be when I'm in Photoshop where I'll, 1) save the TIFF file, 2) resize the TIFF file to the size I want, 3) save as a JPEG, and 4) discard any changes to the original TIFF if it asks me. TIFF is an uncompressed format.
Resizing as you are involves, 1) compressing the TIFF file information as you convert to JPEG, and 2) modifying a compressed version of the file. Depending on your compression/quality level, the more compressed/lower the quality the more you lose with each conversion. Heck, if you have a JPEG file with a quality setting of 60, each time you save the file you lose additional information as it recompresses on each save (try it - the file size will get smaller and smaller and smaller with no other changes to the the image).