dithering JRiver output down to 18 bits

Mike - Thanks for writing with the suggestion.   After a bit of work, I agree that TPDF seems to do a very good job.  Here's what I did to determine that.
I installed the free Voxengo Span VST plugin into JRiver and put it at the end of the DSP chain.  Then I generated a 1khz test tone in JRiver and played it back in repeat mode.  I also enabled the "bitdepth simulator" in JRiver, and played the 1khz file at 32, 24, and 16 bits and I even went down to 4 to see how messy it would get without dithering.  Then I set the bitdepth simulator to 18 bits, checked "dither" (TPDF) and the graph was very clean, so this is what I'm going to use.
It took a while to get this figured out but it does seem that using the bitdepth simulator in JRiver is a good solution.
Dave

I am not -- the plugins I've used do only 16 bits, 24 bits, 32 bits. Still, I would like to offer related advice.

When you find your plugin, if it offers several types of dither, listen before you decide which to use. I did some recording a few years ago with decent equipment. When the time came to dither to 16 bits, I noticed added fizziness in the vocals. I had used "noise-shaped" dither. When I switched to plain old TPF dither, the artifacts disappeared. YMMV, but do listen before you decide. Some modern improvements have side effects.