Yeah, makes no sense to put that little battery operated filter between the Monique and amp. And, unfortunately as you found, a buffer is a buffer and once you put that device between your bass and amp, your bass is 'active'.
While the FDECK is a useful device, I find the Monique's bass control is sensitive enough for me to find a very nice balance between a big low end and 'overextending the drivers' (i.e., too much pistoning due to sub bass). So, see if you can get away without it.
I again find that sensitivity control much more impactful with a passive bass, which is beautiful. Given that part of its effect is impedance, this makes total sense to me. The good news is that a passive P or J doesn't send significant sub bass into the system, compared, for example, to my active Roscoe, which can get quite unruly down there. Of course, putting another active buffer in front of an already buffered signal would have little impact, and the entire function of the sensitivity control is much more subtle anyway with an active signal chain.
So, no FDECK with passive and FDECK with active seems to be a good rule for me.