Sonus Faber vrs Magnepan

I know little about the Maggies and nothing about how Blue Circle might work with SF's, but can say that the Signum is a special speaker with unusually good sound at its price point. It was designed for maximum transient response, and uses an unconventional surround/suspension on the mid-woofer that makes the driver really fast. Assuming you do not require deep bass or have a large room, it is welcome in top systems. Aside from the Guarneri, it is the best small SF monitor, and is unfortunately now out of production. It requires quality amplification (but not a lot of power).

If you want a more full-range speaker that can do orchestral music and rock, the Extrema is the best in the line (and some would argue, the best SF ever made other than the Stradavari), but it is also unfortunately out of production and requires a beefy amp. The Electa Amator is a nice speaker, but less distinguished.

I have limited familiarity with Sonus Faber, but am well
acquainted with Magnepan. Comparing the two is sort of
like comparing a dolphin to a horse. Being a conventional
box/cabinet speaker it will be quite different in sound
from a planar. I've owned a number of conventional speakers
including Spendor, Genesis, Thiel and many others. In my
experience they can all sound good but unlike the planar
sound which is unique. Personally I'm sold on planars but
positioning can be problematic for some. They demand space
behind the speaker (3 feet minimum in most cases) and do
better with more distance to the rear wall.

I love the Maggie/planar sound too. I've lived with the 3.6 and Duetta Sigs for 8 yrs now. Recently I needed a speaker for a smaller room. I went through alot of name brand monitors trying to find something that would work in this room. Nothing worked until I tried the Omega Super 3r. It's the model the owner, Louis, recommended for me. He's a Quad and Maggie lover also. I've had it a year now and am very happy with it. The Signums were on my short list as I heard they can work well in problem rooms, but I found the Omegas before I had a chance to listen, and I've always lusted after a pair of Guarneri's. Good luck.