Anything as " fast" as SPECTRAL gear?

02-06-15: Bifwynne
So Al, what does that mean in the context of ARC ref amps. As I mentioned above, the slew rate for my amp (150 wpc) is a paltry 13 volts/microsecond and the rise time is a sluggish 2 microseconds. The top of the line Ref 750 has a slew rate of 20 volts/microsecond and a rise time of 1.5 microseconds.

Bifwynne, doing the math for your ARC Ref 150 tube amp -
if the amp is outputting its full/rated 150W into 8 Ohms that would mean a RMS voltage of 34.6Vrms of a sinusoidal waveform & a peak of approx 49Vp of the same sine waveform. I.E. the peak-peak voltage of the sine wave would be 98Vpp. If amp traversed the 98Vpp such that the slew rate of the amp is 13V/us (as spec'd) then the maximum frequency that the amp could handle with this slew rate would be 66KHz. So, it appears that the freq BW of your amp is 20Hz (or 5Hz?) - 66KHz.
Compared to the (fictitious?) example Almarg gave where the amp was a 200W unit & had a slew rate of atleast 45V/us such that it could handle a signal as high as 200KHz.
So, the ARC Ref 150 has quite a bit lower slew rate compared the Almarg's example power amp & accordingly lower frequency bandwidth.
This also means that since the ARC Ref 150 bandwidth is just 3X (rather than 8X or 10X) the music bandwidth (of 20KHz) one can expect to hear the amp impart its own phase shift onto the higher frequencies of the music. This can manifest itself in a few ways - the highs could sound rolled-off or they could sound warmer or there could be less sparkle/shimmer compared to an amp of higher bandwidth. Nothing wrong with this sort of attribute of a power amp - many like it & many others don't. Something keep in mind.
Just some additional info, FWIW.

I note, btw, that the ARC Ref 150 has a specified 3 db bandwidth of 120 kHz (at 1 watt, with an 80 kHz "power bandwidth" also being specified), and JA's measurements indicate a bandwidth of approximately 100 kHz (for an 8 ohm load connected to the 8 ohm tap), with excellent reproduction of a 10 kHz square wave (under small signal conditions, as noted under the graph).

Hi Almarg, thanks for the feedback.
I did also note the very same specs as you wrote above & also noted that these were mostly under small-signal conditions while slew-rate is a large-signal parameter (as you already noted in an earlier post). When the amp is outputting a lot more power then i believe that the distortion will be much higher & the various BWs cited in JA's measurements will constrict & that 10KHz sq wave won't look so pretty anymore . That's why a 66KHz BW at max rated power under the stated 13V/us slew rate did not seem unreasonable to me. The effect of the excess phase shift would thus be more discernible (vs. lower power outputs).

I think Quiznos makes a much better sub - too bad that they didn't have their biz model under control. In my area, they have all but disappeared....

BTW, re "the two most technically knowledgeable," let's not overlook Bombaywalla, who is certainly no slouch when it comes to EE technical matters, and perhaps some of the others who have responded as well.

thank you, Almarg, for the acknowledgement. :-)