You seem very knowledgable on HTC phones. Can I ask what S-off is? I was going to use this tool to flash recovery. As far I as know, I do not have to do anything with the bootloader since its already unlocked. I was under the impression once I flash a custom recovery, I can install any radio I want? Do HTC phones not allow the flashing of modems/radios?
Only thing to keep in mind, is that since all security checks are off, this includes some critical safeguards like device check. So you need to be careful that you only flash mods intended for the M8. Not difficult if you stay in the proper device forum. But I've seen instances on my old One X (EVITA) where some folks flashed mods intended for the ENDEAVORU (also called the One X, but completely different chipset hardware) and bricked the phone. In the S-on condition, the device check would have failed, and prevented the mod from being flashed, therefore saving the phone from being bricked.
I personally do not recommend using any toolkits to mod the phone. They seem like a quick and easy way to get things done. But in doing so, you are robbing yourself of a valuable learning opportunity. Knowledge that if often valuable when things don't go quite right (which is practically inevitable, even for the most careful of users) when modding the phone.
In particular, flashing a custom recovery is simple as long as you have adb/fastboot installed. Which is also highly recommended, as they are valuable troubleshooting tools. If you have adb/fastboot setup, installing recovery is as simple as placing the recovery file in the fastboot folder on your compter, opening command prompt, and typing the single command (with phone connected):
fastboot flash recovery "filename.img"
Where "filename.img" is the actual file name of the recovery (with no quotes).
For links to s-off process, custom recoveries, radio thread, and other useful resources, check out my Index thread: http://forum.xda-developers.com/show....php?t=2751432