Suggest an adequate mono amp for subwoofer

You can get there for a lot less than $1k. A classic Hafler DH-500 that will pump 255wpc@ 8 ohms in stereo mode and can deliver up to 800W in bridged mono. The DH-500 can deliver over 20 amperes into a short circuit! Do a web search for the legendary reputation the amp has.

I used one to run both my subwoofers and it had no problem.

They run $250-350 on the used market.

I think a Bryston 4B-ST will give you 400 watts in bridged mode. There's a couple for sale on audiogon for <$1000.
Check on the specs to see if I'm correct.