John, I also have a PreSonus Studio 24C on my Resolve. its a nice I/O.
I'm on MacOS so its not entirely the same as you as on Mac Studio 24C uses the native OS core audio, no driver needed. With the monitor gain on full, dim off and no 'normalisation', my output speaker level does sound a little less than from my Studio One out but its small, no where near 30dB.
ASIO support or not shouldn't be the issue. We don't have it. But there should be no reason why you cant select the in the preferences as you have and have the level similar if not the same. If there is a big level difference in the Windows setting that Resolve uses, as charles pointed out, then id like if u could to verify that.
Does this Windows Application level adjustment get you closer if its set full?
Finally, are you on 16.2.1?
Charles Bennett wrote:Have a look here. With Resolve running you can see that the output defaults to a lower level.