I am a bit late in replying, but my experience in comparing ABW with QTR is the same as that of Brian, i.e. ABS is far from linear.
I compared step wedges printed with the Epson 7880 using ABW and QTR on Epson Enhanced matte, both with Warm tone selected. I then scanned the step wedges with an iOne and ran the data through QTR Linearize Data. As you can see, the QTR print is almost perfectly linear and requires no correction, whereas the ABW print needs a significant curve to make the output completely linear.
The A and B values are so similar it looks to me that both ABC-Warm and QTR-Warm are using only all gray inks, MK, LK and LLK.
I posted the QTR Linearize .out data in the zzMisc folder in the Files section if anyone is interested in looking at them.
Sandy