I built the BK-16 flat pack a few months back with the FF165 drivers. I don't have a SET right now and use it with a Quickie and my Fisher SA100. I am finishing up a RH84-se for these right now (and a Foreplay and some Dynaco MKIII's -- to many fires burning, but I love it). I have also heard the BK-20 at a friends place with a SET amp using 6V6's.
I love the presentation of my BK16. I am a real stickler for well defined mid-range, especially in the 800hz to 1Khz range where lower harmonics and middle fundamentals begin to collide. And where I think the beautiful honkiness of a Les Paul lives. Maybe this is due to the quick response of the FF165, other people here more wise and experienced could probably explain that better. Now, I don't have a lot of experience with full range drivers, but I also now understand some of the criticism I have read.
Robben Ford's Cut Me To the Bone enveloped me and most of Karajan's Beethoven symphonies are astonishingly clear, vivid and powerful with exceptional dynamics. Some of the better recorded Dead shows I have absolutely set the entire stage right in front of me. The depth of the piano on one was so convincing that for the piano to my left, the high keys were closer to me and the lower ones were further back towards the wall -- just like the piano was in the room. I have only ever heard that phenomena once before from some Magnepans driven by a Mac tube pre and a big Mac SS and playing some great Brubeck.
But I have some Vivaldi that sounded like it was coming from a stock car stereo and some Beatles and Cream that was a little shrill.
One thing that shocked me with the BK-16's was how much bass it actually had. It wasn't like a sub or a great full range, but much better than I expected. Which makes me interested in trying out the Decware 6" to compare against the Fostex. The heartbeats on Dark Side of the Moon had some nice punch and almost the tiniest bit of rumble. Most importantly, it was pleasing and I didn't want to stop listening.
I am very intrigued by the Decware 8" for a taller folded horn design for when I decide to build something with 45's, 2A3's, 50's or 300B.
Still searching for a perfect match for my Fisher SA100. I am going to build a Two PI tower next to try for that amp -- then a Four Pi