I haven't played the new model yet, but I have sometimes pulled tubes from other 100W amps, usually to protect low-rated cabinets. 100 watts just sounds better to me, at any volume. It's not the power (I wouldn't normally crank a 50W amp right up, either), it's the depth and richness of the tone... I'm pretty sure it's because there are four power tubes instead of two (ie two on each side of the push-pull power amp) - even in a supposedly matched quad there are differences, and it seems to add a bit of extra complexity.
If you're switching the same amp between the two power settings, you should also be aware of the impedance matching issue. I don't know if this is mentioned in the manual for that amp (it is for some of the others) but if you're going to run the amp only at 50W you should use the output jack for half the impedance of the speakers, eg 4 ohms for an 8-ohm cab. If you do that you may find the 50W setting sounds much closer to the 100W.