I hope this helps someone. I was on the latest OTA from Sprint with HBOOT 2.49 - and i found literally nothing to help me having S-OFF, until i did this on my own.
First thing - first! I'm not responsible if you brick your phone - i just share my experience and if somebody find it's useful, then use it.
Things you will need:
1. Download everything you need - Some ADB tool, custom recovery (my phone didn't like CWM so i used TWRP 18.104.22.168), Custom ROM that is already rooted and can be installed while S-ON. Open a new tab in your browser with Firewater's page so you can follow the instructions later.
2. Unlock your bootloader with HTCDEV.
3. Install custom recovery and make a backup.
4. Install custom rom (ViperMax did the job for me, it's not a rom preference, it's a workhorse of choice). Now you have root.
5. Boot the phone and set it up for WiFi, check USB Debugging, uncheck fast boot in power menu, allow mock locations, uncheck apps verifying via USB.
6. Follow instructions on Firewater's page.
7. You are now S-OFF!!! (my phone needed 3 bottles for s-off, so be patient).
If you have any questions i will be happy to reply! Good luck!