Admittedly, I am not familiar with Atkinson's test involving an IHF speaker load. Care to explain what that test is all about?Start with http://www.stereophile.com/content/audio-research-reference-150-power-amplifier-measurements and then read the loudspeaker measurement section of some loudspeakers. Please don't take this as my endorsement of Stereophile's reviewers in general . . . but specifically Atkinson's measurement sidebars tend to be very consistent an well-reasoned, and he explains himself well.
in the case of the ARC Ref 150 which uses 14 db of NF and probably has an output impedance between .4 and 1 ohm (probably closer to .4 ohm because its "rated" and reported DF is 17), can one assume that the Ref 150 will behave "SS-like" if presented with a speaker load that was voiced to be driven by a high current SS amp?In the above link, you'll see that Atkinson actually measured the output impedance as 1 - 1.4 ohms from the 8-ohm output tap, corresponding to a Damping Factor of 8 - 5.7 respectively. This amp actually has a somewhat higher output impedance than most of the high-quality, correctly-operating vintage tube amps that I've measured . . . most of which actually have lower output impedances than many modern "low-feedback" solid-state designs. I think with the ARC Ref 150, the interaction between the amplifier's output impedance and most loudspeakers' impedance curves will be a significant part of its sonic signature.
Short of shlepping a 75 pound tube amp to a dealer to properly audition speakers, it's helpful to know (or at least be able to reasonably predict) if a tube amp can "switch-hit" and function in a "SS-like" way if presented with a speaker load which was voiced to be driven by a low-impedance, high DF, high-current SS amp.The loudspeaker/amplifier equation is complex, and schlepping an amp to the dealer (or demoing the speakers in your home) is absolutely the best way to be sure you'll be happy with it. After all, can you really study automotive specifications and get an idea whether or not you'll like a car without at least putting forth the effort to take a serious test drive?
If you really don't want to pair up your amplifier with speakers you're considering, and you want to be happy with the way they work together . . . then I feel the most logical solution is to purchase the amplifier, loudspeakers, and loudspeaker cables all together, or purchase active speakers.