Yes (we can also use them in our preamps). One of the driver tubes has a little more voltage on it, in your case the one that is at the edge of the chassis facing the power tubes. That one has to have either a -GTA or -GTB suffix if you use NOS; you can also use the Chinese tubes we use and most of the Russians too (although most of them do not hold up well).
It seems that a number of the boutique tubes don't have the higher voltage rating, so you can use them in all of the locations in our equipment except the one described.
It seems that a number of the boutique tubes don't have the higher voltage rating, so you can use them in all of the locations in our equipment except the one described.