They're right (though their use of the term 'warmer' is misleading though not untrue). The point I'm making is that the amount of bass is constant. All that changes is the amount of treble (and possibly some mids, depending on cap value). Notice that that quote mentions bleeding off treble but not bleeding off bass - 250Ks are warmer, because they lack treble, not because 500Ks lack bass.
The exact same effect that you desire can be achieved by rolling off the tone knob vs changing the pots and keeping the tone at 10. Just trying to save you some time and money and prevent people from thinking that pot values can reduce the amount of bass.
Then again, maybe you want a tone even bassier than that, and have not only switched the pot but also rolled the tone off all the way, in which case I'd recomend playing with cap values to change the rolloff point of the tone knob and make it more usefull for your purposes. That's still cheaper than buying new pots, and will allow more versatility (you can have varrying degrees of that treble back should the need arise).