I was finally able to get where I needed to be, but it was quite convoluted. For anyone else who finds themselves in the position I was in, which was running the GPE firmware 3.06.1700.10, with CID T-MOB010, here's what I had to do to get updated firmware with s-off.
I used Windows 7 Pro 64-bit with SP1. Disabled Antivirus and Windows Firewall.
Wipe device and install Guru_Reset_M7_1.27.531.11.zip from this thread
, choose full wipe, restore stock recovery, don't root.
Boot system, it will be stock T-Mobile Android 4.1.1.
Go into system updates. Install the HDR Camera fix only. Reboot.
Go into system updates again. Install the system update 3.24.531.3. It will take some time. If it reboots and fails the first time, just re-run the update until it completes. It took 3 times on my device.
Boot up, now you'll be in Android 4.3, with firmware to match the T-Mobile CID (x.x.531.x).
Next, I ran the RUU 3.24.531.3 from this thread
. Make sure you have the latest HTC Sync software installed.
After running the RUU, my device was back to stock T-Mobile 3.24.531.3 with hboot 1.55, Android 4.3 with Sense 5. Boot up and do the first run setup.
Uninstall HTC Sync software completely. Reboot Windows.
Make sure adb detects your device properly.
Now, I ran the 3.24.531.3 Rumrunner from this download page
. It poured 3 times and was successful. Yes, it was successful on hboot 1.55!!!
Now I have successful s-off with hboot 1.55, latest T-Mobile firmware and radio.
Then I reset the tampered flag, changed my cid to 11111111 supercid, unlocked my bootloader again, and updated the firmware to 3.62.401.1.
See attached photo (sorry for low resolution, that's the best my tablet can do).