How many tube watts for Vandersteen 2's?

Hi Pabul57. I am not sure about Rogers comments with regards light loading however,it is impossible to ascertain with any certainty which tap is appropriate to use as no loudspeaker represents an unchanging impedance at all frequencies.
The 8 ohm tap typically match loads between 4 and 8 ohms, the 4 ohm tap will be a better match for impedances that fall below 4 ohms.

The one thing that I am more than certain of..... The 8 ohm tap is far and away the right impedance choice for the Merlin.

If you use the 4 ohm tap on an amplifier with a speaker of higher impedance, the output transformer will be inadequately loaded, and so it will express less of its winding ratio and more of its inter-winding capacitance. This can result in the amplifier no longer having flat frequency response. In addition, the transformer can 'ring' if inadequately loaded, which is another way of saying that it will distort.

The Merlin is an 8 ohm load, with a dip to 6 ohms or so. Its a benign load and an amplifier with an output transformer, if the transformer is designed properly, will likely work best on the 8 ohm tap. This will minimize the artifact of the transformer.

Modjeski comments on his RM10:

"The amplifier is flat within 0.1dB and has low distortion of 0.3% when played below clipping on average level material. At the recommended bias current of 30mA/pair, the idling dissipation is nine watts or 75% of the tubes' rating. I estimate tube life to be 5,000 to 10,000 hours. Although higher idling currents will reduce distortion, it can also be reduced by light loading. Basically, light loading reduces the output current demand on the output tubes, allowing them to be more linear. It also reduces noise, raises damping factor, reduces distortion by 78% and allows for 80% more peak current when needed. The only loss is about 20% of the power rating or 1dB." Light loading means connecting the speaker on the tap that's one half its nominal impedance rating (i.e. the 4-ohm tap for 8-ohm speakers). For 4-ohm speakers, the he recommends running two RM-10s bridged to 70-watt monoblocks.

In my limited experience w/my new RM 10, the light loading seems to have the best sound w/my speakers which are around 8 ohm min. impedance & 92dB. It's the best audio purchase I've made to date.

I prefer the light loading too, but I do wonder why Ecclectique has had a consistent preference with using the 8 ohm tap. That of course can be explained depending on the speaker, but he did say he tried it with Merlin, I'm using the VSMs.