Not really volume. More bits of resolution means that an analog signal is broken into smaller pieces, but that is the greatest benefit comes from the analog-to-digital process (when you take that analog source and convert it to digital). Your CD's are 16-bit no matter what your DAC can do. You can get 24-bit from DVD-Audio and Blu-ray. You aren't going to get a 32-bit source. I would be curious to know what the practical benefits of a 32-bit DAC are. Barring some sort of upsampling to 32-bit, I am just not sure what value exists (aside from perhaps marketing hand-waving).

The hardware design for the 997 and R-972 was probably finalized quite a while ago. If it has any similarities to the Model 990's analog section, it will use a Cirrus DAC. I don't know that Cirrus has a 32-bit DAC yet, and I'm sure they didn't when the R-972 hardware design was finalized. TI's Burr-Brown brand appears to have started offering a 32-bit DAC pretty recently. Since Onkyo was already using Burr-Brown DAC's, it was probably easy to switch over when they developed the new model year.