In my experience, tube rolling with the Herron is possible but difficult. The biggest problem is that the supplied tubes are extremely low noise, and most other tubes can't meet that low noise standard. And I'm sure the VTPH-2A is voiced with those specific tubes.
The one tube I've had success with, though it doesn't really change the character of the sound in any meaningful way, are hand selected new stock Mullard CV4004(12AX7) from Upscale (Kevin's stash) and Mullard ECC81(12AT7) also Kevin's stash. I tried some NOS Telefunken Diamond 12AX7's I had lying around, but they were much noisier than the aforementioned tubes. Though I think I'll just stick with the stock tubes when it comes time to replace them.
The one tube I've had success with, though it doesn't really change the character of the sound in any meaningful way, are hand selected new stock Mullard CV4004(12AX7) from Upscale (Kevin's stash) and Mullard ECC81(12AT7) also Kevin's stash. I tried some NOS Telefunken Diamond 12AX7's I had lying around, but they were much noisier than the aforementioned tubes. Though I think I'll just stick with the stock tubes when it comes time to replace them.