Why would you say that?
Im running my orion hcca 2 channel class AB amps at 1 ohm mono.
Also, i put my 2150XTR in my buddies car powering a jl 12w7 at 3 ohm and it does it just fine (and its only rated 4 ohm stable)....

Almost true, The bigger the box, the less efficient the sub will be. It's like comparing it to a small sealed box and free-air. The small sealed box can blow the sub very easily because it can't move as much as it wants to. The free-air (or over-sized box) is lees possible to blow the sub because it has less back-pressure.
There are many box programs out there that will support my theory.
As for the suggested box size from factory will typically protect their sub from you blowing it up more that give you optimum output. Nobody wants to warranty their shit.

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"The small sealed box can blow the sub very easily because it can't move as much as it wants to. The free-air (or over-sized box) is lees possible to blow the sub because it has less back-pressure." ....."As for the suggested box size from factory will typically protect their sub from you blowing it up more that give you optimum output. Nobody wants to warranty their shit."

can you clarify that, it appears to be a contradiction. If the factory specs are there to protect the sub, then they should be recommending free air and the largest box according to your claim that, "...small sealed boxes can blow the sub...." As far as I'm aware, most subs can handle more power in a sealed box (or a ported box above it's tuning) than in a free air or large box set up. I can tell you first hand that I've blown a few subs in free air with very little power compared to what they were getting in a box of reasonable size. If you want to see a cone flopping around like fish out of water, mount it free air and feed it lots of power.

Not all amps are created equally. The HCCA amps are just an awesome amp!

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Pioneer Premier DEH-P800PRS
Pioneer Premier TS-A 520PRS
Pioneer Premier TS-A 720PRS
Digital Designs M3
Digital Designs S4
Digital Designs A2 for DD tweeters in my dash
Digital Designs 9110d x 2
Streetwires makes my world go around
Hushmat keeps it real
Kinetik Batteries keep it running
142db in an SQ Sentra.

SQMonster needs to do more research, as he is 180 degrees wrong. I agree with alphadawg. As far as running 3 ohms on a 4 ohm stable amp it may work ok, but running 1 ohm on a 4 ohm stable amp is pretty unbelievable. The whole point is reliability, and 9/10 times this combo will fail almost immediately, unless you are not pushing your system at all. Plus you gotta consider your damping factor at that point, and make sure it is high enough for that particular amp to run at such low impedance, as the sound may be very muddy, but some people don't even care, cause they just want it louder, not clearer.

So is my best bet to go AB 2 ohm stable or D 2 ohm stable? To play it safe... or even 1 ohm stable....

that's fine that it does that, which many amps will, but to use a product that's not designed for a particular application is wrong! It invites possible product failure at anytime, and certainly should void any warranty that is given. The problem with using products outside their designed parameters is that you never know how the consumer will use the product. You may be treating your Orion amps mildly and they're working fine outside their range, but you have no idea how the next person will use their's. If you're willing to take a chance on your equipment, and void any warranty or are prepared to repair it shoud it fail, then by all means try it. Just don't blame the manufacturer for a crappy product when it's you that used it incorrectly!

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You need to know what final impedance will be on your set-up. I am assuming you already know, that your sub will be wired to 2 ohms. It is hard to find a 2-ohm stable 2-channel amp in mono(bridged mode). If you get a 1-ohm stable amp, which most of the time means it gives full power at 1 ohm, so you won't be using the amp's full potential at 2 ohms, as that type of amp will put out partial power at that impedance.

So, with information at hand, I would suggest looking for a Class D amp, which gives full power and is stable at 2 ohm. Unless you can get your hands on a regulated power supply amp, which puts out equal power at 1-4 ohms, but they usually cost a bit more.

But wait, I just read that you are planning to use a 3-ohm single voice coil sub(wonder what the sub is...) Usually, running an unregulated D amp at 3 ohms will not put out full potential power, so if thats the case, then you should be fine with any good quality 2-channel amp that is stable to 4-ohm bridged mono mode, as running it at 3 ohms should be ok.

A nice regulated amp, that you will be more than happy with is JL audio slash series, and with this amp you will no longer care about impedance issues. It is not the cheapest amp, but a lot of people are extremely happy with it, and chances are you will not need to upgrade from it, as it is a killer amp already, and not too much stuff out there is better(debatable). Another amp to consider is Arc Audio KS Class H monoblock, or the older XXK series. All these you can find used.

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FOR SALE: ^^^subs(1.33 ohms) in custom wedged box(1" front MDF) + acoustic wool inside. $450 No offers. Amazing, mint, but too much weight for my lowered car.

All this is entirely wrong. It seems you may have googled stuff, but misinterpreted all of it, and then you are giving out wrong advice, and setting newbies on the wrong track. It seems you may not have enough hands on experience, or otherwise, you would not be saying these physically defiant theories.

Don't get it the wrong way, I am not trying to shame you, and maybe you just wrote it wrong I have seen many car audio gurus write bs, but they admitted thay were intoxicated or very tired when wrote it lol. No hard feelings man!

Skeedy, there are also some Class AB monoblocks out there as well which are 2 ohm stable. DLS come to mind...