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Most amps do have a HPF built in that is not listed in specs or manuals. They're usually somewhere between 30-40 hz. There are amps that have them set higher tho.
On Page 3 through AgedHorse made it sound like that wasn't the case, though I may be mis-reading his post. If so though, I think that was crazyBassClown's point. Why have two? I suppose if the built in one is set too low to have any real use, a second one set higher could help, but why would the designers have set it there in the first place? If they know that an HPF is helpful, then surely they'd know approximately where to optimally set it for the sound they want.
The one reason that it is not generally recommended to use a bass control in place of a HPF is that, generally, bass controls are set at frequencies high enough to have more negative effects on the sound than benefits. You also have to remember that the change you make to the bass control effects every other filter that operates in the range that is adjusted by that bass control. If you have a hof built into the amp set at 35hz, then boost the bass by 6db, now instead of the signal being -3db at 35, it is roughly +3db. This may or may not be a "problem".However, when you have a HPF you can set higher, while you boost the bass control, you can keep the subsonics and extreme lows cut, while gaining the benefit of boosted bass frequencies. The opinions of those who say this is the better way are based on experience trying both ways and listening to the results. It almost always SOUNDS better to use the hpf instead of trying to use a semi-parametric eq.
Usually because of cost/space limitations. Most amp makers are supplying a generic product to a large group of people. As has been mentioned here, many, if not most, bass players do not even know what a hpf is, why include one as a "feature" when so many players still think a bass knob should be at 10 and then blame a cab for being junk if it farts out.
I'm not sure if you are a car guy, but why do many car enthusiasts modify different engine/powertrain/exhaust or even audio equipment when the designer designed the car a specific way already? The same logic applies.
To feed the machine. You can't just sell one product and have the customer be happy. You need to sell a base product to entice them into upgrading and accessorizing!
Calling something "hype" is a troll's errand at best and fake news at worst especially since there is a long history of development and use regarding the topic on TalkBass and in the industry.
For those interested in fact based information and application of H/LPFs, checkout my: TB Wiki on High Pass and Low Pass Filters
There's also my High Pass Filter (HPF) and Low Pass Filter (LPF) Mega Thread that covers the breadth and depth of the topic and contains many, many posts about successful applications and details of H/LPFs. Also included are links to many other threads on the topic, including this one.
IMO, Mesa has progressed amp topology by including both a fixed HPF a 30hz (to protect the speakers) and an adjustable one to tame stage and room boom when needed. For those of us who understand what H/LPFs can and can't do and without built-in and/or adjustable H/LPFs in our amps, units available for individual purchase have been the best piece of kit added to our rigs, in pretty much forever.
For those interested in fact based information and application of H/LPFs, checkout my: TB Wiki on High Pass and Low Pass Filters
There's also my High Pass Filter (HPF) and Low Pass Filter (LPF) Mega Thread that covers the breadth and depth of the topic and contains many, many posts about successful applications and details of H/LPFs. Also included are links to many other threads on the topic, including this one.
IMO, Mesa has progressed amp topology by including both a fixed HPF a 30hz (to protect the speakers) and an adjustable one to tame stage and room boom when needed. For those of us who understand what H/LPFs can and can't do and without built-in and/or adjustable H/LPFs in our amps, units available for individual purchase have been the best piece of kit added to our rigs, in pretty much forever.
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So there are definitely conflicting thoughts here. Some are saying that a basic HPF is built in to most commercial amps and it's just not marketed and another camp that says the most amps do NOT have an HPF built in because the average consumer doesn't care (my argument here would be that if it sounds better, saves the amp a bit on headroom, and costs very little, won't the customer be happier in the long run even if they're unaware of the feature?) So which is it? I don't know that super productive non-theoretical conversation can happen without knowing the current state of things.
It is impossible to know without comfirmation from the maker, it being shown in a block diagram, or opening the amp and looking at the design. I do know the Fender Rumble 800 has a 2nd order shown in the block diagram.
A wonderful quick thread, thanks! So per usual, the truth is somewhere in the middle and the ageless question of "Will this help" is answered as always with "It depends." Some amps have a built-in (marketed or non-marketed) HPF and buying an external one may be a waste of money while others don't have one built in, to our knowledge, and adding a HPF would very likely be advantageous as the benefits of a properly designed HPF is well documented.
I am not suggesting anyone open their amp as this will usually void the warranty...
While many amps have a built in HPF, for the vast majority of them, it's a fixed frequency HPF thats set pretty far off the deep end frequency wise. 30hz seems pretty common.
For me, the sweepable Frequency LPF and HPF are worth the extra money and pedal board space. I can tweak the sound to what I like, instead of what the manufacturer decided protected the speakers best.
It also gives me more precise control of my tone, and allows for on the fly adjustments to account for a boomy room, extra noise, and a bunch of scenarios that a fixed HPF won't help with.
For me, the sweepable Frequency LPF and HPF are worth the extra money and pedal board space. I can tweak the sound to what I like, instead of what the manufacturer decided protected the speakers best.
It also gives me more precise control of my tone, and allows for on the fly adjustments to account for a boomy room, extra noise, and a bunch of scenarios that a fixed HPF won't help with.
I would say the one situation where a hpf will have the least impact is with a sealed enclosure. Sealed enclosures have a natural rolloff that often keeps the speaker in control and also keeps the sound more clear. There are still benefits to using a hpf with a sealed cab, but often times they are less audible.
Sealed cabs still react to stage and room boom, one of the areas where an adjustable HPF can overcome poor venue acoustics and still leave your amp eq/tone controls available for use.
A pre-made HPF costs only a few tens of Dollars. Just pick up the proper tool for what you need and be happy. 
I would think that one of these will give even more precise control of your tone in many ways, especially the critical midrange(note the 400,500,800 grouping) and when combined with the built in 30hz filter allows a lot of control of low end rolloff by manipulation of the bottom few sliders. YMMV though.
In reality I use one of these though. I find the cut at 62 and boost at 125 give me exactly the curve I am looking for, no hpf required. Never used one of the newer GEB7 though. Maybe I just like brown
And a final comment about amps with built in HPF, my query was how many had been designed by AH without a hpf, his reply none, presumably none without. Confusion due to my poor choice of wording for the question I guess.
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