Mosfets & Hybrid amps = not equal. On one hand Mosfets are actually "pentodes" when it comes to the transfer curve. So, to that extent they are "tubish." They also tend to have a soft saturation, meaning that they tend to clip gracefully, not as soft as tubes, but not hard and sudden like BJTs.
No amp is any "faster" than the source that it has to reproduce - which in our case is fairly pathetic these days, being bandwidth limited like a brick wall at 20kHz. Having said that, there is some benefit to a bandwidth on the order of 250-500kHz. and the resulting slew rate that comes along with that.
The sound of "fast" amps, IMHO, has little to do with the nominal specs - it has more to do with the distortion products and how it handles things like leading edge damping in the realm of real world loads. The power supply design and size plays a great role in the way a given amp will sound - all other things being about equal and the circuit being well executed and reasonably designed (not always the case...)
Hybrid amps, perhaps the original being the Moscode, usually have a tube in the front end. The *only* purpose for this is to provide some degree of euphonic coloration. Which, it can. So, if you *want* that slightly "tubey" coloration (not a bad thing in many cases) for your system, one way to get that PLUS a reasonable DF, and power is to use a Mosfet output stage. The only caveat is that the Mosfets want a fair amount of drive power, so there should be something between the tube and the mosfet to drive the output stage. Usually there is...
Personally, I'd get a rock stable, solid, low distortion, high current, high bias class AB FET input Mosfet output amp and "adjust" any desired coloration in a line stage, if that is what I wanted to acheive. My Symphony No.1 amplifier is an example of such a design... you can hear the effect of whatever you put in front of it, very nicely. Makes it easy to set up a system, not having to worry about the "color" of an amplifier in the equation. (IMHO)
As far as OTLs, nice, if you get them to be stable, if you can control the leading edge, and IF you have *enough* output tubes to provide a sufficiently low output Z and high enough current *for the load you have to drive.* But they have nothing to do with Mosfets...
No amp is any "faster" than the source that it has to reproduce - which in our case is fairly pathetic these days, being bandwidth limited like a brick wall at 20kHz. Having said that, there is some benefit to a bandwidth on the order of 250-500kHz. and the resulting slew rate that comes along with that.
The sound of "fast" amps, IMHO, has little to do with the nominal specs - it has more to do with the distortion products and how it handles things like leading edge damping in the realm of real world loads. The power supply design and size plays a great role in the way a given amp will sound - all other things being about equal and the circuit being well executed and reasonably designed (not always the case...)
Hybrid amps, perhaps the original being the Moscode, usually have a tube in the front end. The *only* purpose for this is to provide some degree of euphonic coloration. Which, it can. So, if you *want* that slightly "tubey" coloration (not a bad thing in many cases) for your system, one way to get that PLUS a reasonable DF, and power is to use a Mosfet output stage. The only caveat is that the Mosfets want a fair amount of drive power, so there should be something between the tube and the mosfet to drive the output stage. Usually there is...
Personally, I'd get a rock stable, solid, low distortion, high current, high bias class AB FET input Mosfet output amp and "adjust" any desired coloration in a line stage, if that is what I wanted to acheive. My Symphony No.1 amplifier is an example of such a design... you can hear the effect of whatever you put in front of it, very nicely. Makes it easy to set up a system, not having to worry about the "color" of an amplifier in the equation. (IMHO)
As far as OTLs, nice, if you get them to be stable, if you can control the leading edge, and IF you have *enough* output tubes to provide a sufficiently low output Z and high enough current *for the load you have to drive.* But they have nothing to do with Mosfets...