In fact passives are usually better if they are full up, as there is less shunt resistance to ground, so the source (if a weak tube output) sees an even easier load, which equals better dynamics for them.
Passives sound just the same down low as they do up high, so long as the source is not a weak high impedance tube output stage >1kohm
These two contradict, both written by you. So which is it?
My experience mirrors the former rather than the latter (we used to build a passive years ago and continue to test them); all (and I do mean all) weaken the bass as you turn it down from full. For our testing we used a CDP with a solid state output; it is by no means 'weak'.
Some of this of course depends on the source- for example if an output coupling cap is involved (which it almost always is). But its all moot if you have a preamp as it buffers the input from the output cable.
