Well, that’s a new one on me. The 390pf comprised part of the original feedback scheme, and my guess is that it may have been removed in concert with the changes in the coupling cap values as part of an effort to "improve" on a commonly believed issue with the Stereo 70’s bandwidth inconsistencies specifically in relation to the feedback loop/coupling caps.
The thing is, our respected friend, Dave Gillespie, has recently done an extensive study of the original Stereo 70 design and finds that some of the commonly held beliefs about deficiencies in the original design are unfounded. He also addressed the matter of the coupling cap values and the feedback loop, and I believe he felt that the original parts’ values were reasonable and that increasing the coupling cap values could possibly result in instability under some circumstances, or so I understand.
Here are a couple of links, the first to his overall study and the second to a discussion of the feedback loop and the coupling cap values, both of which seem relevant to your situation:
Unless I could find something validating the rationale for removal of the 390pf, and if everything else in the amp is configured according to the original design, I’d be inclined to put the 390pf back in and change the resistor back to whatever the original value and power rating was.
BTW, it has been my understanding that the 390pf needs to be a 1000VDC part: http://www.audioasylum.com/forums/dynaco/messages/6416.html I found Joe's comments to be true when the corresponding part in my Stereo 35 went. It was quite a show.
Mouser Part # 598-CDV16FF391JO3