passive volume pedals are like adding an extra volume pot to your guitar, so they load your tone a bit if you have passive pickups (which you don't).... but the different values will have different loading characteristics and a different sweep.... some are better for doing swell and reverse effects, some are easier to nail in on very specific spots so you can have a couple different output levels when playing dynamics aren't enough
that said, passive volume pedals are generally kidna delicate, moreso than a traditional wah pedal... I have had better luck with the morley optical volume pedals, though they have their delicate parts too
the ebst volume epdal I've ever tried was the visual sound active version with the LED indicator for output level, its built like a brick shithouse and is a pretty neutral buffer/booster type circuit that can actually enhance your tone depending on what your definition of nice treble response is
personally I just use my volume knobs, why have 'em on there if you don't use 'em? I flirted with pedals but its just another thing to keep track of while performing and another gadget that can break down mid-set
if you are just looking for 2 preset levels (say half and full output) you could build a stompbox with a passive volume attenuation when engaged, if you perceive trable loss try different value pots ranging from 250k to 1meg in conjunction with playing with treble-bleed networks... in my arsenal I have a 3 voice booster I designed that's an always on tone enhancer/shaper with 2 different footswitchable output levels, one with treble bleed so tis bright at low output and one with a lower value pot and no bleed so it provides a louder and somewhat darker tone when turned up as a boost... its a very simple circuit, but a smidge more complicated than a straight passive volume-knob in a project box with a footswitch
there's a pile of info, but I am just not super clear on what you are trying to do so it may all be worthless to you