Uh, to be honest, I think QC15 qualifies as "warm sound signature and great sound isolation in the 300$ range".
The answer will be how important is noise canceling for you dad.
If so, QC15 is the best on the market for comfortness, noise canceling, and sound among noise canceling phones.
One of my own experience may be useful for your purpose.
I didn't find myself to use my QC15 very often besides traveling.
It's no really because that I have better...