Since you are concerned specifically about the noise and distortion on the preouts, you should refer to the following graphs for answers:
So you can see that if you believe you can tell the difference between 87 dB and 95 dB SINAD at 2 V, you should go for the Cinema 40.
In case you are more used to distortions and noise in %, 87 dB vs 95 dB is approx. 0.0045% vs 0.0018%
To per things in perspective, the Marantz AVP, model AV7705, at 2 V preout (RCA, or 4 V XLR) is about -75 dB, that is about 0.018% and that is obviously many times higher than that of the AVR-X3800H's 0.0045%.
It is important to note that such comparisons may not mean as much as it seems because for example, the results were for test signal of 1 kHz and at 2 V. So, if you listen at a very loud level of say, 10 dB below reference, the preout voltage may vary between 0 and 1 V, and may never get close to 2 V, depending on the sensitivity of your speakers and you distance. If you know the distance, and your speaker's sensitivity are known, you can easily estimate the maximum pre out voltage you need for the buckeyeamp.
Specifically, based on your 13 ft distance and the ELX's sensitivity of 90.5 dB (though I would de-rate it to say 88 dB such to be on the conservative side as they do seem closer to 5-6 ohms than the 8 ohms Ascend claimed, you may need pre outs voltage close to 1.5 V during media contents peaks. In that case, even the X3800H's distortions + noise levels wouldn't be an issue.
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Another important point is, the performance between the two AVRs seemed to be much closer if you compare the above graphs for Amir's test results but for "wide band" signal, take a look:
The difference, was only about 2-3 dB, to the Denon's favor. In other words, in this measurements, that is more relevant to real world use, the Denon won. The bottom line is, Amir's top line comments are often based on one or more specific tests that is obviously useful for evaluating the duts in terms of "engineering excellence" but not always relevant (though often enough) to purchase decision making that we want to base on potential "sound quality (whatever the means)".
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Based on distortions+noise comparisons above, if you do listen to near reference level at 13 ft, there won't be much difference between using the X3800H and the C40, though if the price are about the same then for sure the Cinema 40 would be a great buy. Here in the USA or Canada, even the Cinema 50 would cost much more than the AVR-X3800H so I don't understand why you can get the C40 for about the same price.
If the X6800H costs just a little more, then in my opinion it is a better value. There's no measurements available, but given that it has the same DAC chip as the Cinema 30, I am confident that it would have much lower distortions and noise than the Cinema 40.
If money is the most important factor, I would vote for the Cinema 40, otherwise the X6800H is the best of the 3 based on specs and projected bench test measurements. I don't have "professional" opinion, but that's my opinion based on my experience in this hobby, and mostly on available specs and measurements.
I also think you should not freak out as there are no practical reasons for the difference between those 3 units to be noticeable, even if you listen to reference level. At that level, you may feel enough pain, to quickly turn the volume down at least 5 to 10 clicks lol..
roon.app
And it also eventually costs you more, than your favourite music service you already pay for.
