After doing some extensive IR recording and captures on the Quad Cortex these last couple of days, I definitely hear a difference in the QC captures or IR due to the 21ms being too short.
The symptoms are the same whether it is an IR or a capture (I suspect captures are truncated to 21ms too)
It’s less apparent for “immediate sounding” amp heads or cabs but obvious on gear that has a “bloom” quality to them (pretty subjective words in quotes here).
My open back custom cab with 2x12 has its natural low end missing on the IRs imported (and trucated) on the QC. Btw, it is not room generated low-end, it’s really the cab doing its thing.
My mid gain amp heads with a bit of sag sound the same as above.
I really believe it is as wrong to say 20ms is all we need for replicating a piece of gear as it was when people said 320 mp3s were just as good as CDs (remember those years ?).
I do encourage people to test their IRs with clean or mid gain and play around with the IR length (not on the QC but on a IR loader in order to play around with the length/truncation).
As a quick footnote, NDSP plugins carry longer IRs which makes them sound a lot better (and a lot more amp-like) than the QC.
I hope NDSP make the decision (because that’s all it is) to allow longer IRs…