I followed the steps in the post linked in the first reply (used Method 2, Method 1 kept failing fastboot halfway through) and reset my phone back to stock. But it still doesn't want to connect to Sprint's LTE. I even played around with the LTE radio settings again in the hope of enabling Spark and nada.
Would it have been possible to corrupt the phone software totally while doing this to begin with? Do I need to send it back? (and on that note, you have no idea how frustrating it is to explain to a Sprint rep on the phone that you bought the phone from Google online. Such a foreign concept to them!) Could the SIM card have gotten corrupted and I just need a new one of those?
EVO 4G >> EVO 3D >> EVO 4G LTE >> NEXUS 5
ROM: PAC ROM 4.4.3 (with Hover) or some other Kit Kat goodness
Unlocked via a slight breeze