Yes, consider it done - in the frame of the pending (and long overdue ) review of the iFi Audio Pro iESL . I have used an extremely good amp from 1977 ( plus or minus a year or two - max ) - power amp section of Technics SU-8080 integrated amplifier ( which is nothing but an improved version - completely separate power supplies for each channel - of SE-9060 power amp ) to drive the iFi unit. For reference Stax SRM1MK2 amplifier has been used - both combos driving a single pair of Stax Lambda Pro headphones. I did throw additional capacitance of the order of magnitude of another pair for measurements. You'll find everything in that review - ARTA
http://www.artalabs.hr/ scren shots of THD, IMD, etc.
I am well aware of the danger of >20 kHz signals messing up with the amps. Been for ages. Absolutely NOTHING can inject a higher frequency garbage into the signal than a phono cartridge on the verge of mistracking - it is even worse than at the actual mistracking. FAR too high in frequency for any normal digital measurement equipment ( short of "officially we don't have it, but can measure in a pinch" military stuff ) to be displayed in the graph or "oscilloscope screen shot ". I had to get a 100 MHz analog oscilloscope to see it - as 20 unit MHz was at the very edge of "showing somethhing".
Going for so high extension in the frequency response IS a double edged sword. I use AGI 511 preamplifier
http://www.audiogeneral.com/. The man who designed it in cca 1974 is Mr. Donald Siegel - and he is THE originator of the double blind AB(X) testing. It is STILL the fastest (phono) preamp - ever. As Mr. Siegel is obviously not exactly keen on promotion etc ( he is LONG ago moved from audio only to his current main objective, furnishing the complete A/V installations for halls, etc ), I will not publish the original brochure or even the circuit diagram - which both are a model of precision, if there ever was one in audio. Here a brief summary from Japan :
http://audio-database.com/AGI/amp/model511-e.html Main point is that the bandwidth for the phono section is - minimum - 90 MHz ( NOT a typo, in words : ninety megahertz, minimum ). It is the only phono preamp adhering to RIAA curve ( deviating not more than 4 mB ( again, NOT a misprint, mili Bell ) - from the ideal - built with near zero tolerance parts in the RLC filter; i.e; a resistor of say 23865 ohm shown in circuit diagram will not be 24 kohm 0.X% part, but HAS 23865 PRINTED on it ... ) that can re-constitute the next to perfect square wave fed to its input trough an inverse RIAA network. Every other phono preamp I have ever tested this way does round the leading edge of the square wave - if ever so slightly.
Likewise, the line stage does similar - for the low frequencies. Into 47 kohm load, its output is less than 3 dB down below at 0.1 Hz - and can drive 20 Hz - flat - into 600 ohms load ... It has the most accurate 20 Hz square wave response of any AC coupled preamplifier.
Now, AGI 511 is ANYTHING BUT a consumer amplifier. It is built better than any Hewlett Packard or Tektronix measurement equipment I have ever seen. In many ways, it is still unmatched to this day - and if upgraded trough what technology ( new generation IC op amps have 1/10th of distortion and noise and are at least two times faster than the original complement ) did bring in the 4 decades after its introduction, it is a VERY tough partner in a shootout - for any preamp. Its Aichile's heel(s) is (are) purely mechanical; the RCA jacks that interface directly with a very dedicated RCA printed circuit board have a nasty propensity to break signal carrying contact - and they are a major PITA to repair. I lucked out a few years ago, finding the original RCAs on ebay - but that is more fluke luck than anything else. Replacing all the RCAs with something modern and making a PCB to fit the chassis is a MAJOR operation - and costly. Also, the potentiometers used are intentionally carbon type - chosen for audio quality. That means that tracking between the two channels never was really good even in a brand new unit - and can not be expected to retain the same quality 40+ years later. No noise or intermittent sound, anywhere within the operation range of the potentiometers, though. Luckily, AGI 511 is not a slimline pancake - so enough place to fit either a potentiometer or stepped attenuator as volume and balance control$$$$ .
For all of the above reasons, AGI 511 can be your best friend - or worst enemy, depending how you look at it. It will unearth - AUDIBLY - stylus wear well before any other preamp would. The extreme case happened at one of my friends , who used a well regarded tube preamp at the time. He has been complaining that lately his beloved Koetsu cartridge started to sound "strange". OK, an oscilloscope and AGI 511 ( line stage ) are always with me in such cases. A quick glance at the output from the Koetsu playing back square wave at innermost radius on the test record showed - CLEARLY - that the stylus is worn beyond usability. Playing back the normal music using said unnamed tubed preamp it sounded "strange" - but playing back trough AGI 511 it was - unlistenable.
That is WHY I also use other, lesser phono preamps; used records can have too much damage inflicted to them troughout the decades, even if now played back by a VdH/Micro Line type of stylus - and filtering some of it out may well sound advantageous.
The point is the fact that my friend has been listening to ( and irreparably destroying irreplaceable records...) with a stylus that was well past both its prime and usability - because the preamp has been "nice" and swept all the garbage generated by the worn stylus under the carpet - trough excessive HF filtering.
My ( and not mine only ...) regret is that Mr. Siegel did not decide to put the accompaniying power amp prototype to the market. The thing was rumored to have 1000 V per microsecond slew rate - in second half of the 1970s . It was reportedly too expensive to produce due to extremely tight semiconductor tolerances required. I have never even seen the picture of the unit - but judging from the 511 preamp, it must have been a truly awesome amp.
Yes, CDs do allow for the lesser amplifiers to be used to decent effect - but in 2018, there are good ways known to build an amp that does not cost an arm and a leg and can still manage the >20 kHz HF content without excessive distress.
But, we do agree on one point; the recording in question has nothing above say 30 kHz that is meaningful information on the ORIGINAL recording - which back in the day meant analogue reel to reel tape. The only true use of HR digital is in the new recordings of either new musicians and/or new music. As of today, there are - at least - two microphones available for music recordings that go to at least 100 kHz - and it is about time "recorders" catch up.
