Mismatched tubes degrade performance and if badly mismatched, sound quality. Tube characteristics vary over a statistical bell curve. It is more probable in a sample of several hundred or thousand, they would appear close to what the datasheet says about them. The tolerances of tube characteristics are on the loose side and therefore the fewer of them there are, the more the standard deviations become apparent from the mean (I'm doing my best to communicate this here
). In an absolute worse case scenario, the bias current would be upset pretty severely between the two. I definitely would go for matched tubes having said all that.
The MKIII was marketed as a mono unit. In the early 60's people were going to stereo. I would guess the general technical ignorance of most Americans would not have allowed them to envision two separate amplifiers acting as stereo. Therefore they bought was was sold to them as "stereo" by salesmen and advertising. Therefore most people went with the ST70 as it was a low cost unit, that despite its relatively low output power, was stereo.
Therefore my opinion on the MKIII and it not being as ubiquitous as the Stereo 70 was cost, technical shortsightedness, and marketing. After all, why would anyone buy something "mono" in the age of stereo?
If I were around in the 1960's I would have went with dual mono because I would have been equipped with the insight that dual mono = stereo. Plus I would have liked like the extra power!
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