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

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.

A nice QSC amp would do wonders to a sub!


    

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 )