Greetings John from Central FL!
I confirmed that the thumbDV is at /dev/ttyUSB0. Here's the dmesg output:
225.880433] usb 1-2: FTDI USB Serial Device converter now attached to ttyUSB0

I ran the test python test program with success (after editing to change the serial device to /dev/ttyUSB0):
python ./AMBEtest3.py
Setting ...
/dev/ttyUSB0
d
8
N
1
False
False
False

Reset
6100010033
Wrote: 5 bytes

Product ID
6100010030
Wrote: 5 bytes

Version
6100010031
Wrote: 5 bytes

Set DSTAR Mode
61000c000a013007634000000000000048
Wrote: 17 bytes

I then tried starting AMBEserver from the command line (without luck apparently...):
sudo AMBEserver -s 460800 -p 2460 -i /dev/ttyUSB0
AMBEserver: Starting...
AMBEserver: Opened serial port /dev/ttyUSB0 at 460800 bps.
AMBEserver: Couldn't find start byte in serial data
AMBEserver: error receiving response to reset
AMBEserver: Could not initialize the DV3K!

John, NX2I