FWIW on all my Bisonics I find the tone sufficiently different between full on volume and rolled back to about 7 that I use both. At 7 they are somewhat less cluttered and more focused if those words suggest a sound. When I am playing melodic lines, such as cello transcriptions, I find the individual notes are much better defined and it is easier to diagnose my mistakes.
where is your tone knob during all of this?
also turning the vol down is cutting the string signal that the pickup is "picking up." I do not personally play bass loud enough to mess around with that. the tone pot tapers the high/low frequencies, but you are working with the entire string signal, so that part is a constant.
Derek Trucks uses the vol knob on his guitar as an effect, he can only do that and sound that way because his amps are on full blast. some older Allman shows u can really hear how loud his guitar is at the top end of those volume swells and solos. people don't understand how they can't get someone else's tone...just because they have the same guitar and rig...well, try turning it up until the walls of your house shake...then it should sound right.
other players use a volume pedal to achive a similar effect to Trucks, but they are never cutting the string signal, they are just tapering the output (volume) of the singnal coming out of the electronics of the guitar, into the amp.
also back to the original point of this thread, im not familliar with the OP amp (Rumble 200) but it seems to have raving reviews. im not really an electronics guy but i got my Thunderbass head tuned up by someone that knew what they were doing (Jay at Ant Farm) and now bass sounds unreal thru that! i've also played an Ampeg tube bass amp, at GC a while ago, and had an Orange Terror 500 with a pre amp tube- all sounded super warm and punchy. u may want to explore demoing an old bass tube amp like a Thunderbass. u might be able to somehow use it as a pre amp or something. that would warm things up/round things out for sure. your current amp does have great reviews though. how does it sound with a Fender (solid body) bass?
i dont think caps and pots/wiring make any real noticable difference, just in themselves. it may somehow "sound a little better," which could mean a lot of things, but the tone will be the same. the Newark Street basses sound pretty good/fantastic (whatever the spectrum of opinions are) right out of the box.