Hi Scotty,
In listening, I'm not sure it's that big of a deal in practice - For amps with sufficient output transformer core capacity and primary inductance (which provides the coupling you speak of), this distortion shouldn't become audible (>.5%) until relatively high powers, at frequencies which don't come into play too often. This bass distortion will be audible for typical "little" tube amplifiers (say, a vintage tube receiver, or a single ended amp) driving too-big speakers. But for a McIntosh, Audio Research, or Citation, much less-so. Since loudspeaker distortion tends to be highest in this range, too, the effects are generally not too detrimental or obvious. For serious bass applications, using an active crossover with a solid state amp driving a subwoofer would be a smart improvement, though.
At reasonable powers, many tube amps can pull off a nice 10Hz sine wave with distortion that isn't appalling, so it is often a cost limitation more than anything. Output transformers involve trade offs, and a bigger core does not necessarily mean better overall performance without a fancier coil winding. A lower-power output transformer is easier (cheaper) to build to perform well at high frequencies, due to reduced parasitics. So, there are always compromises to worry about. A do-it-all 100W output transformer might cost 3x as much as an "average" 100W output transformer.
(IMHO) In most cases a damping factor of 10 or higher (output impedance <0.8R for ~8R load), or even 5 or higher, will be sufficient to minimize the frequency response variations to levels where they become negligible compared to other factors. The AR amp definitely achieves its damping factor via negative feedback. To have low output impedance, some form of feedback is necessary, although it doesn't need to be a global feedback loop. Triode push-pull output stages have fairly good damping, as triodes have intrinsic negative feedback that is otherwise "removed" in a pentode. A pentode output stage using cathode feedback or "local" feedback of some sort can also be excellent (McIntosh and Quad use this approach). Before additional feedback (say, a big fat global loop), these approaches might lead to an initial damping factor of ~4. Much better than an ultralinear amp with DF~1 before feedback (look at measurements of a Prima Luna on stereophile!) The vintage classics, which DO use a lot of feedback, typically aimed for a DF~10-20 - McIntosh, Williamson, Citation, Marantz etc. But, damping factor isn't the only figure of merit - stability into a real load is also VERY important - and higher DF means more feedback, which can jeopardize stability.
So, if I were to provide some basic tips for tube-amp-objectivists, given what measurements you might find available - I'd look for:
- A damping factor of 5 or greater
- Enough juice to never exceed 0.5% THD in listening on the speakers of interest
- A stable, reasonably-wiggle-free square wave at 1kHz and 10kHz
- Good (-1dB) bandwidth to at least 40kHz, and down to 10Hz, with no visible "peaks" at high frequencies
- THD+noise <0.1% at 1 Watt @1kHz - and low THD+noise at lower powers, too
- Semi-affordable tubes that are appropriate to application
- Good design (beyond fancy aluminum or chrome), and substantial transformers
- Snake-oil free
Of course, I'd still want to listen to it, and there is some art to interpreting the plots
But from detailed measurements, a basic schematic, and some internals pics, I think it's possible to form a
guesstimate of how a tube amplifier might sound.
A sampling of commercial amps that measure (by my standards) quite well, that can be found on Stereophile's web site: Quicksilver Horn Mono, McIntosh MC2000, Music Reference RM200, Audio Research. I could list some iffy ones, too, but I'll resist
