I have no experience with 7581A. I have tried multiple brands of 6v6, preferring Mazda and RCA blackplate. I am currently running 6F6. The ones I have are little less powerful than 6v6, but also seem a little "sweeter" sounding. One reason I tried them to begin with was ST shape, then really enjoyed the sound and they were cheap, so been running them exclusively for about 9-10 months.

Yes, I think the Coke bottle shape is cool, I have those for my OA3's too & I think the ST shaped power tube and regulator look cool next to each other. Seem to put off a little more light too, but maybe it's because the 6F6 ST shape reflects the OA3 more than GT shape.

Your logic seems sound on the 7581A. I'd probably wait and see if someone more knowledgeable than me can confirm your reasoning, but it sounds like it might be worth trying.