Do all Passive preamps exhibit this trait

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.

I may have disovered the trouble. For the past two weeks I have been using my Krell 300cd player with single ended interconnects. Previously (for 6 years) I had been using it balanced. With a full-level 1kHz sinewave, the output from the 300cd is 4V from the balanced outputs, 2V from the unbalanced. I'm would say the output is too low unbalanced to make a satisfactory match with this passive preamp.