I am going to put on my flak jacket, suit of armor and helmet before I say anything.
Should work.
Merely being logical (if that) about the issue, a few thoughts...
... the signal level will be the same as it is now...
and that will likely be about what the tube spec sheet calls for to run the tube in class C... in turn, there is doubtless a tube before the 813 that serves the purpose of getting the signal level and power up to where it needs to be.
Even if you could swing enough voltage out of a DDS, imo, you would absolutely need to buffer it in order to protect the DDS from over voltage and transients. So depending on what that looks like, the voltage out from the DDS, you'd need either a tube follower or moderate gain tube to drive the 813.
I shall now crawl back into the cave, shut the door behind me, and hide.
Thank you.
_-_-bear