Your question has been answered multiple times in multiple ways. Surely you realise this, yet you keep asking again and again.
There are many variables in terms of the way a particular amp topology is implemented - from solder quality, to capacitor quality, to circuit board layout to power supply. If you need any proof that this is the case up and down a product stack, a better example than the i3/i7 earlier is between different implementations of the ESS 9038Q2M and 9038 Pro: the Topping D50S, with a single 9038Q2M (the 9038 Pro's poor, lower-power cousin intended for portable devices), tests markedly better in terms of measurable, quantitative outcomes than a number of DACs using multiple 9038 Pro chips, which, on paper, should have substantially lower levels of distortion and noise. But they don't, because of the sum of the parts isn't well implemented relative to the better-executed D50S. It's also why a 9028 Pro-based Benchmark DAC3 outperforms most 9038 Pro-based implementations. By the same token, you can't say that because the THX 888 is a higher-grade topology, therefore any amp implementing it will measure (and/or sound) better than the 887 or 789 - this is even more true of (good) solid-state amps than DACs, because there's far less basis for subjectivity, their job is simply to take a signal and make it louder.
The difference between the SNR and THD+N figures of the 789 and the Benchmark are within the margin of error of most audio analysers. More importantly, those slight measurement differences in favour of the Benchmark occur at a level that is well beyond any (again, any) human ear's ability to comprehend them. Think of an audio analyser as similar to a magnifying glass - you can't see pixels on screen beyond a certain dpi, so the difference between, say, 3840x2160 and 5120x2880, in a 6" phone screen, would be impossible to observe. You may be able to observe the difference through a magnifying glass, but doing so would make no difference whatsoever to your qualitative experience. You can say 'more' is 'better', but it's not so in any way that's even remotely tangible as far as your experience as a consumer is concerned. Similarly, other factors in the screen's implementation entirely beyond its resolution (panel type, brightness, contrast ratio, HDR support, etc.) make a far greater difference than a pixel density that you can't distinguish without a magnifying glass. The same is the case here, you (or anyone else) will never be able to hear the sonic differences between a well implemented 789 and 888, in this case the Drop and Benchmark.
In terms of tangible sonic (rather than feature) differences, other than what @twoloszyn mentioned re the volume pots (not something I've observed, but entirely possible), the Benchmark can drive more power into a pair of headphones. That's about it. Again, this is meaningless if the relative maximum outputs are well beyond what it would take to deafen someone. This depends entirely on the headphones you intend on using with the amplifier and how hard they are to drive. You can work easily work out the maximum SPL for a given headphone and amp combination... but honestly, unless you're intending on jumping straight into something like a Susvara or Abyss, the 789 will be more than enough amp, particularly if you are running it balanced. For some context, the 789 will (theoretically), power any of the HD600/650s, HD800S, Utopia, HEKse, 600-ohm Beyers and so on, well in excess of 120db SPL, with THD+N levels that are literally an order of magnitude better than any DAC that may feed it. Let me say that again, the distortion and noise either of these amps layers into your signal is less than 1/10th as much as the distortion and noise fed in by your DAC. Even if it's a really, really good DAC. It's like arguing your Ferrari has a top speed of 329km/h while your mate's Lambo only does 327... but both of you only ever drive on 100km/h roads.
As someone who has spent far too much disposable income on this hobby over the last few years, honestly, get the 789... or an 887, or an SP200. You may very well find it's all the amp you ever need (there's literally a handful of headphones that could do with the Benchmark's extra power, and they're all $3000+). Even if it's not, you'll be far better served spending the additional $2600 between the 789 and HPA4 on a good DAC (start with a DX7 Pro) and a pair of Utopias for now. You need to understand that the whole point of the AAA amps is to provide a 'wire with gain' - in other words a pure, unadulterated and uncolored amplified output of whatever it's sent. Both the 789 and HPA4 do that admirably, to a level of accuracy that exceeds the capability of most DACs they'll ever be connected to. If you're looking for an amplifier to colour your sound, make it sound different or 'better', you're looking at the wrong thing.
On one final note, you're all wrong on the Hugo2. It's easily the best bit of audio gear i own.