Downfiring should work. Nathan Salazar is very happy with his 3.0cf sealed sub, built using 3/4" baltic birch and not heavily braced. He sold me his second SCC300 saying that the one was enough for his room. Several others really like 3.5 cf. I made a spreadsheet of the different cabinet dimensions to get volume between 3.0 and 4.0 cubic feet. I convinced myself that another 1" taller or deeper was not a big deal in the WAF. So, I went 4.0 and it is really big. I recommend you brace it really well with Baltic Birch braces and sandwich of BB/MDF for the side with the driver (baffle) and the opposite side, the others can be MDF.
I found the free cabinet plans in
www.northcreekmusic.com to be very helpful for cabinet design ideas and construction tips. You might want to check out this new sub design, can be used for sub speaker stand (which I remember you being interested, like the BESL's).
http://www.northcreekmusic.com/Thunder/ThunderInfo.htmI went with the Hypex HS200 for plate amp. I was attracted to the active crossover and hi-lo pass filters. The 200 watts is more than enough for the SCC300. Dr. J (Jeff Glowacki) says that even 100 watts (true watts) will be sufficient depending on what you are after.
If I had a nice crossover like the Marchand, I would look for an old Hafler or Adcom amp which will work great mono or stereo if you decide to add a second sub.
Even though the 1801s have very good bass, I like the results much better.
Al