You guys don't mean building the lather IN the tub right? My GFT Rose still has enough on the lid so I just swipe the brush on the lid and bring it to a bowl where I build the lather.
Exactly, you're going to whip up the lather on your face, not in a bowl. Your are just dipping the brush in the cream, instead of putting it on your face with your finger. I treat hard creams, like Acca Kappa 1869, as if they were soaps and lather them in the tub. Do what is best for you. There is no right, or wrong, unless you are not getting a lather.
Maybe we should ask the OP what kind of soap/cream in the "tub" he's talking about. I would imagine that the response would be very different for "Proraso" than it would be for "Trumper" or "TOBS".
Personally, for Proraso in the tub I swirl my brush as though I were loading from a soap puck. For TOBS, I simply dip the tips of the brush into the cream and give a little twist with a fairly dry brush.
I use the 'dip and twist' method. Usually gets me enough lather for 3+ pass shave. Some creams are firmer than others and require a more firm dip but that comes with learning your creams and what you need for your shave.