PASSIVE SUBWOOFER DRIVEN by a 2 CHANNEL PRE-AMP + MONOBLOCK ?

I just saw the website for QSC amps

seems they do stereo amps  and 4 channel amps

I don’t want to bridge a stereo amp so if I just use the one channel the thing won’t blow up in time ?

I assume they are made in USA - do QSC class D amps have a reputation for longevity ? ( my old Jeff Rowland amps that used the bang and olufsen ICE class D amps weren’t very reliable )

Yes, you can adjust the "trim" of the subwoofer output in the HT Processor.  I have never had any problems with some movies having bass/LFE mastered too loud, but if you wanted to put a preamp in the middle, it won't hurt anything.

Also, be aware that most those pro amps (Crown, Behringer, QSC) are fan cooled.  The fans can be relatively loud and will be annoying during movies.  Unless you are putting these amps into a closet or separate room, it may be too much fan noise for your movie experience.  I had a couple older Crown CTS amps and the fan noise got to be too much (almost like hiss coming through the speakers).  The CTS2000 I had did a good job for subwoofer, but the older Emotiva monoblocks were a stronger amp for subwoofer (it didn't matter how loud it was).

Using one channel of a stereo amp is just fine if you want to do this.  Some of these amps, such as the Crown XLS, are Class D amps. 

I would like to put forth another suggestion.  Since you are in Australia, availability of amps is going to be difficult.  I would look at the Nord Acoustics website.  They are in Europe, but ship anywhere.  Look at the "Nord One SE NC1200MB Mono Blocks Single" amp.  These output 400 watts into 8 ohms, but will do up to 1200 watts into a 2 ohm load (which is stable).  It is priced at 1,149 Euro (which is 1,780 Australian dollars).  Contact them and request that it is loaded with the previous generation 8-pin Sparkos SS3602 op amps for the input stage (instead of the SS2590 pro op amps).  These are excellent discrete opamps.  While they don't have the best resolution for high frequencies, they have some of the strongest and fullest bass that I have heard from an op amp (this is mostly due to how the slew rate is handled in the opamp).  I think this solution would be significantly better for your subwoofer than any of the pro amps.

Very interesting reading the Nord website. A lot for your money there . Thanks for the recommendation !

for some reason the engineers sell it with an external amp/crossover module .

Lots of good reasons to do this, including not subjecting the electronics to the rumblings of the inside of the cabinet, and vice-versa.

Inside is a 200 watt class Ab amp.

Which is actually pretty fancy, considering even high-end subwoofers come with Class D most of the time.

To add insult to injury, I noted some made in China capacitors . A disgrace at this price point LOL

The very worst examples of an armchair speaker/electronic buyer are those who throw aspersions without any knowledge at all about what they are talking about. The quality of parts coming from China ranges from the exceptional to poor, like parts in any other place. In fact for many types and ranges you'd be very hard pressed to find a better country of origin than China.

I have not heard this subwoofer, but I think the idea of making an amp specifically with speaker level inputs in mind is quite clever.  I suggest to the OP that his snobbery will lead him only to spending money on peculiar pieces of kit instead of enjoying great audio gear.

I find it hard to believe the OEM would sell an amp that is undersized.   You don't know until you listen.  And yes, many parts are made in China.