And you correct about the “old” mash usage...That is not the case with pure bourbon..It uses only “new” corn mash in it’s distilling processes...Then it is cased in wooden barrels for aging...Sour mash is filtered through ash charcoal before being stored in wooden barrels...
There is a distinct taste difference between bourbon and sour mash...There is a distinct taste difference between rye and bourbon and sour mash.
Pure Canadian rye whiskey is probably the smoothest whiskey ever...
Take a look at the Buffalo Trace website. You'll find a video there about the fermenting process which mentions use of "setback" from a previous batch to start the sour-mash process. Since those guys not only make their own name brand, but also the super-popular straight bourbons sold under the George T. Stagg and Pappy Van Winkle names, I'd say they're representative of how the big "pure" bourbon distillers operate.
Also, look up some of the online scotch reviewers (Ralfy Mitchell, for starters). They're always ranting about the ills of chill-filtration (which sort of does the same thing to scotch that the charcoal-filtering process does to Tennessee whiskey - it sucks part of the flavor out).