You are welcome, just sharing my experience as I had to make the same choice. As you mentioned, there are many, far too many on AVSF that claimed the difference is there, some even said the difference was night and day. Some mentioned the often used expression, that is, it is like lifting a veil off the speakers. Some readers will believe such claims as facts while some would not and may even laughed at such...
I think it is human nature, AVSF members who spent $5K more on the 90 would have all the reasons to believe and therefore actually heard the difference (better that is), whereas those who decided, or happened to pick the 70 would be more likely (just more, ..but they may also have buyer remorse) to think they made the right choice, having spent $5K less, and therefore heard there was no difference if and when they had the opportunity to do their AB comparison listening.
It is not possible to know what people would hear if they had done their comparison using controlled methods such as AB switching without longer than say 1 second day, volume matched, and use the same media contents, players, amps etc., let alone doing it blind or even double blind. I do not recall any of those AVSF members mentioned how they did it and non mentioned doing it blind, or even just level matched, quick AB switching. So I had to base my decision on listening in the dealer's demo room and comparing specs and measurements.
My dealer would love it if I traded in my 70 for the 90, but I never felt there as a veil in front of my speakers, I have since replaced my Bryston and Parasound Halo amp as there were no veil feeling in my two channel systems either even when I compared those more expensive amps with a tiny Fosi V3. There is always a possibility that if my hearing is perfect, I might have a different story, but my instinct tells me there is no likely, because I could easily hear better quality when listening the contents of good recording quality, or between the best MP3 and CD quality, so my hearing, while far from perfect, should be able to tell a difference that could be described of lifting a veil off.
Back to the AVSF comments, the only reasons they presented were something like "the 90 not only has the better DAC, but also more advanced audio circuitry, better parts/components...." all marketing info with no evidence. Even if all factual, are we to believe that using 1% or better tolerance resistors, caps etc., = audible better SQ than using 3% tolerances parts? If so, measurements should show the differences, even if such differences were not audible, but reality is, available measurements didn't show such difference either. Now, I must admit, if I had $5,000 that I could spend on, that otherwise I had no other use, then I would go and trade in my 70 for the 90, but that would be the same reasons why I have had, and still have so many unused DACs, preamps, amps etc...
I hope you know that Anthem typically would put on a significant discount on Black Friday, Boxing day kind of events, at least they had done so at least 2 or 3 times a year in the past 3 years. I bought my when it on sale. Also, I would make sure the dealer agreed to let you trade it in for the 90 within so many days. When I traded my in for the 8K (still the 70), I basically lost about $500 within a month. The reason for that trade was not about sound or video quality, it is more about stupidity, emotion etc.
I listened to both at the dealer's, if I had heard better SQ from the 90, I would have bought it. The 90 does have the flagship class ESS DAC IC, but the SINAD specs between that one and the 70's is virtually the same only 2 dB difference, but the 90's has higher DNR, that is 138 vs 129 dB. 129 dB DNR is already excellent, 9 dB higher looks good on paper, but not worth spending a lot more just to get it. But then ymmv, as always. I do think you will be happy with the 70.
