SeaNile. I know the PB and SB13-Ultras come with high- and low-pass filters. I think the Rythmiks do also, but you need to check. I have an SB13-Ultra in a room about your volume, and it's rather open to other rooms, but not wide open. For music it performs exceptionally well, but so would a Rythmik. For movies, it is surprisingly good, but not in the league of some others mentioned (and not mentioned) here. It's one of a few subs that can pass most WAFs.
Since you only have one seating position, a single sub may work out great. If I were to pick a single sub for that room, I think I'd go for the Rythmik FV15HP as long as they come with the RCA ports needed for you to hook up your Hegel. But the Rythmik is quite large and may not pass the WAF.
There are many companies and subs to choose from: Hsu, PSA, Reaction Audio, Rythmik Audio, SVS, and all have great subs. You need to talk to them on your unique requirements and get a feel of what they have to offer you.