You can roll the tubes - but I don't see the point.
1. The preamps already sound superb. SUPERB.
2. Keith voices the Herron gear incredibly cunningly. He chooses his tubes exceedingly carefully, for reliability as well as fidelity. These things are darned near bombproof. He designs around these specifically chosen tubes, ones that he knows will sound great and give long life without changing sound. They aren't exotic, so you can replace them if you ever need to.
3. Part of the beauty of Herron gear is it's neutral without being colorless or lean - neutral in the best sense of that word i.e. "natural" sounding, just getting the hell outa the way of the music.
4. Although Herron gear is less sensitive to cables and PCs than most tube gear I've had (and that's true of the Herron ss amps too), you'd probably get more mileage for less trouble by replacing the stock cord than rolling the tubes.
5. Every dedicated tube roller I know of that swapped out the stock tubes eventually put 'em back. You look at the humble stock tube and think: "what if I substituted XYZ NOS tube? Must be awesome..." Not. Please see #2 and #3 above.
6. If, after all that, you just have to swap the tubes, talk to Keith first. Really.