I'm not really a "I told you so" kinda guy.
I kind of took your question #2 as asking if you could run off the 4 ohm and 8 ohm taps simultaneously by using the separate 4 and 8 ohm jacks.
Speaker impedance isn't an exacting science, not nearly as cut and dry as the ohm rating of a resistor for example, which is generally a <5% variance from the marked rating.
Speaker impedance ratings are "nominal"; for example an 8 ohm driver can fluctuate from just a couple of ohms to 100+ in actual use depending on frequency, power, cabinet, etc.
But, there is a reason that tube amps have output transformers, and a reason they are tapped off at specific impedances. The output transformer is used to match the load (speaker) impedance to the output stage impedance (tubes) for
optimal performance.
A 2.67 ohm load on your amp
probably won't cause any damage, your tech is right about tube amps being pretty flexible, but the mismatched load can hinder the amp from working up to it's design specification.
Which can be good or bad depending on what you are trying to accomplish. When I was a kid my goal was to be as loud as I could get, and using my basic high school electronics class skills I would hook up as many speakers as I could to whatever amp I could get my hands on with little regard for sound quality. I
understood impedance, but took extensive liberties in pursuit of "loud".
Eventually I figured out that for
me anyway,
quality was a more desired attribute than
quantity.
IMO it's
easy to get loud,
not so easy to sound good doing it.
