This great thread you start Mastah Vu.
When I built my 300B audio amp I used a Heapshit IMD analyzer and a British (can't remember the brand) THD analyzer. That's a good question, voltage or power. These instruments measure voltage across a load, which implies that they measure power. But I guess it all depends on whether they use a 10log scale or a 20log scale. At any rate, .1% is considered Fine Business for an audio amp and most agree that anything under .5% is undetectable by the human ear. However, as we know, most receivers (except 51S-1s and SX-117s) have much more dynamic range than the human ear.
Tom Vu the audiophools definitely know about 6550s, but mostly for replacement stock. Ever price a Tung-Sol 6550? I scored a quad of them at a hamfest for a quarter a piece, I'm counting on them to fund my kid's college.
Like everything associated with Art Collins that SSB book is very pricey. I saw one on ebay a few days ago for $100. I should have invested in real estate in Cedar Rapids; I'm sure the Collins Collectors will discover that next
73 John