@charles1dad - When I first finished the ANK amps, I was using GR-Research NX-Oticas. These cross over to powered subs at around 70Hz. The 300B amps provided a bit more depth and width to the sound stage, a more natural decay, and a smoother midrange. Since I'm not using the amps for the lowest couple octaves, I can't really comment on the deep bass performance differences.
My new speakers (based on the GR-Research Line Force) cross over an octave or so higher (around 170Hz) and are more efficient (98db/w vs 93). With these speakers, the 300B amps are an even better match. The midrange, particularly vocals, sound exquisite.
I started with ANK's top amplifier model - C-Core Interstage parallel 300B SET monoblocks. As I mentioned, I made a lot of upgrades including high-impedance current-mode heater supplies for the 300Bs, regulated DC heater supply for the driver tube, Maida regulator on the driver stage, Audio Note silver tantalums for all resistors, upgraded wiring and connectors, and a couple minor circuit changes. And a much beefier, aluminum chassis. Also upgraded to Western Electric (new issue) 300B tubes.
