I also want to post concerning the use of my fdeck Series 3 HPF.
My findings are exactly the same as BurningSky. I used all new cables, both TRS and TS, between the pre and power and in front of the pre.
Results were exactly the same as BurningSky. I am using the Crest ProLite 2.0 power amp, and my fdeck was purchased around early November of last year.
Currently, I have the fdeck in front of Monique. It is working great in this location of the signal chain.
I just don't feel comfortable with the difference in gain with it in between the pre and power.
No big deal, I justed to post it in case anyone else come across this setup and experiences the same thing.
I ran my HPF3 in between my Monique and my IPR for about a year with the associated unexpected 'signal gain bonus'. In that time I never felt it was dangerous or hazardous.
I used it there primarily to protect my speakers and obviously for the 'room benefits' it provides. I was worried that any errant tube grumbles, crackles, static would potentially damage my speakers, running the IPR bridged at ~3Kw. I also had concern because the Monique does have a wide low shelving bass control (centered ~53hz I believe), and since I boost my lows, I reach pretty low and was seeing some fairly major speaker excursion that didn't seem beneficial to my tone. I also play a bass with a 37" scale length, so I also probably have more fundamental and content down low than many bass players.
Beyond the excursion and transient protection, I also found that since the Monique can push so low, I was engaging the DDT protection on my amp. I did not experience that with my previous 'main' preamp, the Kern IP-777 with similar tone and output settings. Putting the FDeck HPF3 in between the units cleared that up entirely even WITH the impedance mojo we're discussing.
I ended up removing it from between the units and putting it in front of my Monique input for several reasons, one being that the Monique CAN exceed the headroom that the HPF3 has, and can cause some raunchy breakup. I eventually felt that I didn't need to jockey what in my mind is yet another 'gain stage' to keep my sound clean. I already have about 4, with my onboard bass preamp, the Monique Sensitivity control, the Monique output and the power amp.
Funny this should come up now. I'm doing a gig now in Philly at the Crystal Tea Room and my sound guy got a little annoyed with me. He said when I thump on the strings his speaker cones are shaking immensely. Monique puts out a wicked thump. Now I am using an fdeck series 3 but I was using it between Monique and my power amp. So my sound guy was not benefiting from the fdeck because the DI was obviously before it. So this last set I switched it. I put the fdeck in front of Monique to see if that would help.
When I did this I observed a few things. I had to increase the line output on the Monique to about 4pm this way when I used to have it about 3pm. But I did notice a difference in sound too. It seems to have a bit more roundness to the notes and the mids are thicker with more punch if that is even possible! The tone actually reminds me of my Read pre a bit, but much fuller and thicker. I'm still getting a great thump out of it too. I guess sometimes a problem becomes a blessing in disguise. I guess the impedance match is better in front of the Monique than after. Or I guess I just prefer the tone to my ears before Monique. Who knows?? And to think all this time I used it after and it took a problem with my sound guy to make a change. Lol!
Btw, I'm still not sure if it made a difference for my sound guy, but that's his problem now! I'm digging the feel now way too much to change!
I agree. I think it just sounds better out front. After a year of one way, when I moved it, I discerned an improvement of my tone. It may be from running the main out on the Monique full up or nearly so. I don't know why and can't quite describe it...just more better.
Spanky needs to learn where the HPF is on the channel strips...
I agree with this and what Chadds says. One of the reasons I liked having the HPF3 in between Pre/Power was that it gave the soundman unaltered signal from my bass/amp out of the DI. I'd rather provide as 'full' a signal as possible to someone behind the board, then they can decide what they need to do with it. I know some soundmen who DO want to push the subs pretty hard, and I don't want to 'cheat' them of the chance to make whatever mess in the FOH the want.

But really, they should have the tools to handle the tone sculpting they need. They should at barest minimum have a set of limiters and/or a high pass for PA speaker and amp protection.