> My question: I've read and watched videos that recommend the Benchmark HPA4 or the Topping A90, but I'm curious if my ADI-2 DAC FS headphone out is roughly the same as using of these highly-regarded external amps?

There is no point using the hpa4 or a90, unless adi-2 is not loud enough for you.
Adi-2’s hp amp has very low noise and distortion and reaches/exceeds the performance of the DAC.
You won’t get better performance with hpa4 or a90. All three are pretty equivalent in topology or performance
The latter is only better with higher power output.

> Their focus is to make a cheap product and to sell many.

The benchmark hpa4 is certainly not cheap ---  it costs 3 times as much as ADI-2 DAC.
Topping A90 is also super expensive, if you take its BOM, support and feature set into account.
Compared to those two products, ADI-2 DAC is certainly priced very competitively.

I agree with the rest of the points you made, though.

Only two benefits of those two products are 1) more power 2) lower noise, if you use the analog volume control. Especially true with the benchmark HPA4, where volume is controlled via relays, can maintain good channel balance.
In case ADI-2 can already drive your phones loud enough (which is 99% of the case), or you cannot hear any noise with your ADI-2 (also true 99% of the cases),
You gain nothing from those products. You lose, however, useful features such as loudness when adjusting volumes on the HP amp.

“High Power” does not change the amp all, it only switches the analog level after DA-conversion.

The headphone-amp-stage always runs on full, high power.
Therefore you gain better sound quality when using “Auto Reference Level”.

EQ is the key to tailor the bass to your liking, this changes million time more than buying an extra amp.
ADI-2 is top notch and hard to beat anyway.

Store different EQ presets, tweak and store again, compare until you worked your way up to where you want to go.

E.g., for 2 month now I have the Mysphere 3.2 combined with ADI-2, and it’s a dream team.

I worked my way through numerous iterations of EQ, first based on my own measurements, then tweaked again and again during listening.
By comparing the stored EQ presets you can make sure that your next version is a step up against the previous, constantly improving.

The ADI-2 / Mysphere 3.2 combo was good and special from the beginning, but now it’s a great listening experience, unique even within my collection of top-notch headphones.

Once in your life you have to experience a true level matched, blind A/B test of different amplifiers.
I did that quite often, just to be sure, and it’s an ear opener:

The super tiny differences are almost indistinguishable between the amps.
It’s hard to get a hit rate identifying the amps significantly above random.

Opposed to that the Internet audiofool community is full of myths and snake oil.
People hear what they expect to hear by randomly plugging around stuff, then religiously publishing it as the final truth.
Most of what it written isn’t personal experience anyway but collected somewhere, republished again and again until it becomes “common knowledge”: “as everybody knows ...” - argh...

One of those myths is the idea of “power = control” which is again pure imagination, and sometimes technical misunderstanding.
Very few headphones need more than 100 mW to be driven to ear splitting levels.

In fact the only significant and sound changing difference between power amps is the output impedance, the only interaction that happens between amp and headphones.

You have the experience with your OTL tube amplifier which has a much higher output impedance than ADI-2.
In combination with the bass resonance of your headphone’s drivers, this leads to a frequency boost in the upper bass range.
Clearly audible as muddy bass.

“Control” (on a power amp) is a myth and does not exist.

There is output impedance, a technical reality, and it’s frequency response changing effect might be misunderstood as “Control”.

If you like I can explain what exactly happens.
Best you tell me what headphones models you use so I can explain it with your headphones as an example.