Do all Passive preamps exhibit this trait

Being the ONLY person responding who owns and uses the Adcom GPF 750's active and passive in my system...
My other stuff: Forte' 4a amp, B&W 805s speakers (With B&W sand filled stands)Sony SCD777ES AudioResearch phono PH-2 (balanced out)Dual Golden one with a Shure V-15Vmr
Anyway, My experience is that the Adcom in passive mode is 'cleaner' but a bit thin when listening to stuff I want to sound 'full'
When in powered mode, the upper octave gets a bit grainier, the mids bloom a bit, along with the lows.. and the lows get boomier(muddied) , though not louder.
I would say (having owned the Adcom since it came out...) Is that the associated components are what give it the colorations. The adcom in passive is an excellent preamp. It passes what is put in. (cables here are a big deal) in dynamic (powered) mode, it is a decent preamp.
Jazz, Rock, and Classical, all have different needs. The system I have does what I want fairly well.
Also, I use a powerline conditioner. The Adcom is into the Monster 7000SS from an AVS2000. And my amp is from the wall via a adcom power conditioner that only has the amp plugged in.(the amp from the MC7000 is too thin but vary clear. straight from the wall, it is more even, but is not as clean, so the adcom conditioner is a compromise)
The sound from the pre changes with the various ways I had the items connected from the wall, and the power cords I used. The main difference was in the thinness/clarity vs bloom/loss of clarity.
I have only modest cables and powercords.

Thanks Elizabeth for responding. I too feel that the GFP-750 is a fine preamp in passive mode and for the money is a bargain. My only serious complaint with the GFP-750 is better described in this excerpt from an online magazine review:
At various listening sessions, my son Marc, a trained musician for both trombone and guitar, (not an audiophile) would join me and add his input. One CD we listened to was Keith Jarrett’s At The Blue Note: Saturday, June 4, 1994, First Set (ECM1577 78118-21577-2). The trio was beautifully recorded and it became evident that what was missing was the air and extension in the high frequency range, which manifested itself on Jack DeJonette’s delicate and varied cymbal work. To paraphrase Marc, it was as if the decay of the cymbals was cut short. In the bypass mode, this was slight but noticeable and to a lesser degree with Keith Jarrett’s piano. The bass resolution, timbre, and resonance of Gary Peacock’s bass playing were right there with no apparent differences in achieving the full body of the instrument.

Through the Adcom, the dynamics of the piano and the full body of the instrument almost matched the direct feed, but that slight loss of full note decay or ambience was just barely noticeable. And I do mean barely. I feel this is still quite an achievement and more an act of omission than addition of coloration or artificiality. Consider also the fact that an additional set of interconnects and connectors are in the chain. This in itself will cause some losses to the sonic protrait.

Lornoah, I can't comment on the GFP-750, b/c I haven't heard it, but I'm fairly certain it doesn't use transformers or autoformers in passive mode. Potentiometers are the most common attenuator and it wouldn't surprise me if this is what the 750 uses. If this is the case, it would explain why you're hearing these shortcomings. A potentiometer can't preserve the impedance curve of the source when the volume is turned down. Ralph (Atmasphere) explains the technical details up above. But I do know that transformers and autoformers do a much better job at preserving the impedance, and this is why they sound better. You should be able to hear this difference in a resolving system. I certainly did when I compared a Creek OBH-10 (potentiometer-based) to a Bent TVC. The Creek was no match. The Bent had a more open sound and better frequency extension in both directions, but especially in the bass.

Does anyone know of a passive preamp that will provide me with this treble extension and at the same time sounds relaxed with no listener fatigue. Thanks.
Lornoah (Threads)

All of the passives I've owned had excellent frequency extension and no fatigue. There was no premature decay in the upper registers. They presented music with an abundance of air and detail.

If you want to stay with a passive, I recommend a Sonic Euphoria or a Promitheus TVC. They're both common in the classifieds and their tech design is more mature than the 750.

it's a mistake to think that a preamp is not additive or subtractive. since no components are perfect, all are additive and/or subtractive. the colorations of some may be more difficult to detect. after careful listening , i beleiev it is possible to determine the sonic personality of any component.

if you approach component selection with the idea that all have flaws, you will listen until you find that component which complements your sonic preference, without concern for accuracy.

I forgot to mention Dave Slagle's latest DIY passive that Clio09 pointed out. It's based on the same autoformer that John Chapman uses in his current Bent line-up. I absolutely doubt you can do better for $350.