Ribbons. Electrostatics. Planar speakers. Confusio

I would suspect that the passive pre might be part of the problem,but you might try turning the speakers "up" to energize the voice-coils before lowering the volume.Or find a speaker with better low-level resolving power-ribbon/electrostat or 2-way high efficiency speaker.

Resistor based passives present a high impedance to the source at lower volumes. TVC based passives work differently (better, IMO) but are still not ideal loads and require sources of low output impedance (less than 500 ohms). A mismatch in impedance will result in muffled dynamics and would sound bland.

I love my planars but they're not great at low volumes.

There's something called the, "Fletcher-Munson Effect" that defines how one's hearing response varies with acoustic(DB)level. As in: Less sensitive to lows and highs at lower volume levels. That's why some equipment(usually mid-fi) will be found with a, "loudness contour" of some kind. You didn't give much info to go on, with regard to your system. I was using a Placette Passive Linestage, between my Cary SLM-100s and BAT VK-D5 for awhile. The transparency/musicality was wonderful at all volume levels. The passive shouldn't be depriving you of anything. It's probably that the B&Ws aren't the most efficient speakers out there, and don't, "wake up" at low power levels. Planars are usually mid-efficient systems as well(86db/1W/1M, or thereabouts, in general) and (as much as I love them, having owned Acoustat Mod IIIs, and a few Maggies) probably wouldn't avail you much at lower levels. Audition some higher efficiency speakers(90db/1W/1M or better)at low listening levels. There are some fairly efficient systems out there with ribbon-type tweeters. The option is finding a pre that induces no personality of it's own, and yet offers some bass/treble boost at lower listening levels(good luck).

(http://en.wikipedia.org/wiki/Fletcher-Munson_curves) Here's a site that explains the effect I was referring to. Just FYI! If your passive pre were the problem(or it's impedance presented to the source), it would sound the same- regardless of the volume level.

I should qualify that last post. Most resistor based passives(at least- the ones I had researched) are ladder type, and keep a fairly constant resistance presented to the source. Even the Creek(inexpensive) uses an attenuator that presents a constant impedance.