You_Too said:

I'm just wondering about one thing with ResampleHQ: Let's say one was to upscale a perfectly color-reference graded DVD using ResampleHQ for the downscaling after nnedi3. Wouldn't this gamma-aware behavior alter the picture?

Not necessarily. The point is that other downscalers alter the picture by not being gamma-aware. From the ResampleHQ website:

Our eyes don't perceive light linearly. Twice the photons will not appear twice as bright. To account for this, colorspaces with small bit widths (all the ones in Avisynth) are gamma-compressed to give a more eye-friendly representation, ensuring bits are evenly distributed across all perceived intensities.

These gamma-compressed colors must be converted back to a linear colorspace prior to blending, or you get incorrect results. None of Avisynth's filters do this correctly. For rare scenes with a lot of sharp contrast this causes very pronounced errors. For most other scenes the errors are more subtle. It can also effect the quality of anti-aliasing on edges with a lot of contrast (such as in outline-heavy anime and cartoons).

If you're doing color-correction after upscaling/downscaling, then all of this doesn't matter.