Back in my earlier and more adventurous days I bought a pair of Fourier Systems Panthere monoblocks, those with 8 of the Russian military triode 6C33CB tubes in each chassis. They were based on the original Futterman design and were different than anything I tried previously or since. They powered my Wilson Audio Sophias with an enormous soundstage and unmatched transparency, but did not have that lean, hyperdetailed, small-image sound that most other amps have. I would characterize the images as "diffuse". Using a pair of autoformers helped also. However, in addition to doubling as space heaters, drawing 5 amps at idle, they were absolutely unreliable. Sometimes one channel would go down, popping out a little male-shaped red circuit breaker in the back, while on other occasions they sounded less authorative and a check of tube fuses would show several were blown. After investing in Buss fuse company stock I contacted Ralph Karstens and he offered to refurbish the amps but graciously suggested I clean and re-tension all of the tube sockets as they are tempered by the prodigious heat thrown off by the amp. So I had a backyard project with the amps disassembled and parts all over my patio furniture, not to mention an exasperated spouse. I then took it to Brooks Berdan and their technician replaced some diodes and other things I don't understand, and the amps then did become reliable. However, the damage was done and I no longer trusted them and sold them to get something much more reliable. I have never heard nor seen in person any Atma-Sphere equipment, but I did look into these and understand they use a different circuit called a Circlotron which is apparently more reliable, but almost certainly sounds different. I have thought about matching these with my single-driver speakers but reluctant to do so based on previous experience.
