Hi Horacio,
In this case it was me who wasn't quite right, about the 98 and 101 dB. When I wrote my last response I didn't re-read your post which preceded the one I was responding to, so I had forgotten where the 98 dB (at the listening position) had come from.
In any event, as you realize, decreasing load impedance will increasingly challenge an amplifier with respect to the correspondingly increased demand for current (everything else being equal). And also thermally in the case of non-class A designs (SETs are class A), due to the temperature rise caused by the increased current passing through their output circuits.
However, if the amp does not have an output transformer, or for a given output tap if it does have an output transformer (as I assume any 2A3 amp does), if the load impedance rises to high values output power capability will be limited by the amp's voltage swing capability (i.e., the maximum amount of voltage it can put out).
With many solid state amps, that will probably cause the maximum amount of power that can be delivered into 16 ohms to be not a great deal more than 1/2 of what can be delivered into 8 ohms, corresponding to there not being a great deal of voltage headroom relative to the voltage capability required for an 8 ohm load. Some amps that are rated to have several dB or more of dynamic headroom, though, would probably do better than that. Although many times large amounts of dynamic headroom may simply reflect that the amp is not robust enough to sustain high output currents continuously.
With a tube amp operated from its 8 ohm tap, as the load impedance increases significantly above 8 ohms max power capability may initially increase, but will eventually also reach a point where voltage swing capability, and perhaps also increased distortion, will limit its output capability. Where that point occurs will depend on the design of the specific amplifier.
If the amp provides a 16 ohm tap, though, I would generally expect power capability from that tap into 16 ohms to be in the same ballpark as power capability into 8 ohms from the 8 ohm tap.
Best regards,
-- Al