Should I be using a linear color profile or a sRGB IEC61966-2.1 (whatever that means)?
It’s my understanding that you should use linear encoded color for hdr files if used for PBR.
For standard materials you need to be in gamma space, but you can still use a file with linear encoded color values and pass
gammaSpace when creating the cube texture.
I suppose that every renderer in any shading model expects linear RGB. Some picture files such as .png are sRGB gamma encoded by default. Such files are converted to linear color space automatically.
You can unlit materials to be able to compare rendering with the original colors in your textures or solid colors. The difference should be noticable mostly in bright colors.