In an experiment to prove its transparency, Placette actually recommends you placing the unit between your preamp and amps to see if you hear a difference in sound. Then you take out your preamp and then insert it back in between the Placette and amps to see if you now hear a difference. I don't know why you want to put another preamp in your signal chain but I probably would place it after your active preamp and have the Placette attenuate the volume.
Anyways, Like Mike said, as long as the cables connecting the Placette to the amps is short, then you won't have any problems. With a short interconnect from Placette to amps and a long interconnect from source to Placette, your system should sound as it should. But have it the other way around and use a short interconnect from source to Placette and a long interconnect from the Placette to the amps, and you'll suffer the loss of dynamics that is associated with passive preamps.
So judging from the results of my cable experiments described above, it was simply logical and really a requirement for me to place them close to the amps. Lengths from source to the Placette doesn't really matter so much and really minimizes inconveniences as you don't have to worry about keeping your sources close to the Placette.
I'm confident that placing the Placette in between the your active preamp and amps wouldn't do much to the sound, ie. the Placette will be transparent. What I am wondering is whether placing the Placette in between the source and active pre would improve the sound compared to the sound you get without the Placette. I am interested in hearing the results from your own experiments.