active or passive preamp for active speakers

Pick the lowest hanging fruit first...

Passives are pretty cheap and if your system likes it, almost unbeatable.

IME, it is extremely difficult to find a transparent active preamp, I have tried a lot and they all add to the sound too much, or take too much away from the music.

If you want to go active be prepared to spend a lot of money to get a transparent design.

A couple of suggestions: I have a B1 active no gain preamp from First watt, I cannot tell the difference with a passive preamp, which says a lot! No gain, but high output impedance so it turns out it has less drive than a passive...

Placette Active and passives are great.

The Best active preamps I have listened to are The Essential, and the top of the line Metaxas preamp. Amazing both of them and they clearly beat a passive in most ways, including transparency.

While for my taste a passive like the Lightspeed Attenuator (I have tried many others) is better than any active, as Al says it has to be in the right system. It seems that you system may not be ideal, not only because of cable lenghts, but I suspect the SS amp in your speakers are low input impedance. I also think that passives work better with tube amps in general in terms of impedance and sonic character. As you point out, you will certainly have more flexibility using an active preamp - with you current equipment and down the road.

the measured input resistance (DC) for the Dynaudio BM5A was 100Kohms.