How can an active preamp possibly help?

Agree with your observation, and its the same comment that would be made for using a passive preamp -- which is, in effect, the same as not using any preamp except for volume control and source switching. I have gone "passive" and will never go back -- the transparency and overall improvement is astounding, and I don't notice any loss of dynamics or bass, which are some of the arguments which proponents of active preamps make. Presumably, they would also apply those same arguments to your use of the cdp's volume control. If that's the case, why do Mark Levinson and other top names provide a volume control in their cd players. To reduce dynamics and bass? I don't think so.

I've used passive preamps, also found the sound very flat, 2 dimensional, but the bass and top end was very good, transparency great. Going back to good ss active preamp now, offering the best overall performance for my likes. Tried a tube preamp and it didn't 'do it' for me at all. Like anything else, it's something you have to try in your own system to see what works with your tastes, no amount of audiophile jargon or high flow technical theory should sway you any which way, let your ears decide.

Can anyone explain technically why people report any benefits to using an active pre, given my account above?

If the signal is already too loud and needs to be attenuated, how can an active pre possibly add soundstage?

Because many passive pre's cause an impedance mismatch with the output of the CD player, and the input of the amplifier. This causes frequency rolloff problems, and increased distortion. With an active preamp, the inputs and outputs have impedance characteristics that better match the source output impedance and amplifier input impedance, and therefore could provide better sound than going direct, thru a resistive passive pre, even though there is more stuff in the signal path with the active.

However, careful matching of the units, and careful selection of the lengths and type of cables, can mitigate this problem, but may not solve it altogether. The problem can be almost totally overcome by using transformer volume controls/passive pre, that doesn't use resistor volume controls. This would provide the advantages you allude to, and still have a good impedance match on both ends.

So, to fully answer your question, a correctly applied transformer-based passive pre, could give shorter signal path than an active pre, and sufficient gain pass through, while still providing good impedance matches, and likely would provide more transparent sound than an active pre. But an active pre, will more than likely give better sound than a resistive-based passive pre, that is not absolutely perfectly applied, including cable matching. This is primarily due to impedance matching.

Twl, Your post is an important reminder that it is possible to oversimplify with rules of thumb. The "short signal path is best" argument has been made in many posts without considering the signal in context. Many here have sold their preamps only to reintroduce them soon thereafter.