I’m hoping someone can advise me so a sub question
I have an 18 inch Passive subwoofer , the Wilson Benesch Torus

for some reason the engineers sell it with an external amp/crossover module . Inside is a 200 watt class Ab amp. To add insult to injury, I noted some made in China capacitors . A disgrace at this price point LOL

ive never been game to run it loud because I don’t think the amplifier has enough juice

if I experimented , say , using a stereo preamplifier and a 600 watt bryston monoblock and run my XLR cable from the subwoofer out on my home theatre processor —-> to my all analog stereo pre amplifier then only 1 channel on the preamp is being used . Of course the preamp then connect to my Bryston monoblock via an XLR plug

am I potentially doing damage to my preamp by only using one of its channels ?

200 watts seems a tall order to move an 18 inch cone at 20 hz at 80 decibels . I’ve spoken to several techs who feel most of the made in China subwoofer plate amplifiers are rubbish so I’m trying to avoid class D Chinese amps.

the Torus is a good  sub for 2 channel duties I remember reading it’s not that good for home theatre. Thank you if anyone has input.
I’m sure it will be fine!

   More will chime in soon. 
     Many do what your doing, bypassing the cheap bash amp for a better amp.      But your better off with a QSC amp to drive your sub.  My opinion. 
    A nice pro amp will do wonders to a sub!
No, it will not hurt your preamp if you only use one channel.  However, I don't think you need to use a preamp.  The subwoofer output on your HT Processor should already volume controlled (just like all the other channels on the HT Processor output).  You can connect it directly to the Bryston amp.  Although I think there are be better amps.  The Bryston amps tend to be a bit thin sounding on the bass/midbass.

That being said, it looks like your Wilson Benesch is a sealed sub.  These tend to sound a lot cleaner.  For home theater usage, I like to have a ported/vented sub because they sound thicker and more meatier.  If you want to replace the Wilson, maybe look at a Rythmik vented subwoofer.

If you want a really nice sub amp for cheap, look for a used Emotiva XPR-1 "Gen 1" or "Gen 2" amp.  I have found that these have some of the best bass (especially when you upgrade the fuses).  A Parasound JC1 might be okay, but I don't think they are as good in the bass as the Emotiva.  Alternatively, you can look at one of the higher power Class D amps.
Crown, QSC, Behringer (12k) All work well for bass duty.. I really like Behringer 12Ks, 500.00 each. COOL as can be..
Waves in the neighbors pool!!! :-)
Thanks heaps guys . I’ll have to google what is a QSC amplifier and the other recommendations. Much appreciated

yes my Torus 18 inch subwoofer is completely sealed, no ports

I haven’t put a deposit on a Bryston monoblock yet. I only thought of them because after so many friends had their Chinese plate amps fail I thought I’d go with a brand that has a reputation for longevity. But here in Australia Bryston costs an arm and a leg LOL.

Yes my home theatre processor subwoofer XLR out socket can , in theory , be connected direct to a monoblock and then the Wilson Benesch Torus Sub. But the processor has , as one would expect , a global volume knob that adjusts VOLUME of all the speakers plus Sub. 
Auxinput -I think you mean maybe adjust the Trim level in the processor for the sub for a given movie?

i wanted to add a preamp in the chain between processor and monoblock for the sub because some movies have the Bass/ LFE frequencies mastered too loud , other movies the bass is mastered at normal levels. Like in the video world , mixers and studios don’t always adhere to a standard

thanks again guys . I was just reading that even REL has moved manufacturing to China. Sad days.
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. 
I agree with your approach.  I had a plate amplifier die and then bypassed it and ran directly into the woofer from a amp connected to the sub out.

Worked fine. 

Keep experimenting until you are either out of ideas or out of money. 😁

I eventually built a 15 inch with dual passive radiators driven by a McIntosh amp in bridged mode. 

A non vented box gives a smoother frequency curve but not as strong a bass. A tradeoff. Certainly requires more power which is why almost all active subwoofers are vented. 

Vented boxes will have a peak resonance. This can be tuned by changing the length of the port insert tube. 

Experiment and have fun.