I'm far from an expert, so I could be quite wrong; but with a passive tone stack, no matter how it is voiced, its insertion will always incur some kind of cost to the overall signal strength (usually 10-20dB worth). That's why there's a volume boost on the new UA when you hit the switch to bypass the tone stack, even if you have that tone stack dialed in "flat".
Some designs do lose less than others though; the passive James/Baxandall stack might be one of those that loses less, but don't quote me on that.
EDIT: Yeah, hope you didn't quote me on that, the passive E.J. James stack has about 20dB of insertion loss; the original Baxandall doesn't technically have any insertion loss, but that's just because it's essentially an active version of a James tone stack...
EDIT 2: Actually, while the original hi-fi iteration of the James stack has an insertion loss of about 20dB, you can get your losses down to only 7-8dB if you optimize the component values specifically for use with just guitar or bass; so my original recollection wasn't completely off!