After (of course) trying almost everything else first, the trick is that KCS is creating wav files with a much higher volume than the wav files in the 885-1009_tape-I archive, so I had to lower the output volume in the playback device. The square wave output sounds quite different than the "natural" wav files in the archive, so it is hard to do by ear. I used a Mac -based spectrum visualization tool to help.
The recipe is regular mode (not CUTS), 1200 baud. None of the other settings yet, although I'm hopeful that some of the less documented ones will let me tweak the output volume or I can find another automated way to do it.
The command (running in DOSBOX on a Mac) is:
KCS -M -B2 CHASELED.H8T CHASELED.WAV
I want to preserve as much of the functionality, appearance, and user feel of the original equipment as possible. My goal is no modern gear visibly involved. I've built a Arduino -based audio system inside a period-appropriate cassette deck, using it's original functionality -- the deck control buttons and audio/power jacks. The only visible difference is an LED display built into the space where the cassette would normally go to select files with the deck buttons. This lets me replace just the slowly dying magnetic media and keep the original audio interface including the old cables to the computer. It's working for my H8 so far, tweaking for the IMSAI/Tarbell and TRS-80 cassette systems in my collection is next.
Next, I might think about doing the same thing to replace floppy media. Very hard to imagine 8" floppies are going to last forever -- some of my 5" Northstar distribution disks flaked into a big mess a couple of years ago.