I own a pair of the 10” Rocket UFW-10’s in a space a little larger than yours.
They work very well for music, and the parametric EQ was a very big help to get a 20 db room mode bump fixed. The bass is fairly smooth and even now. However, they only hint at shaking the room and there are times I know I’m missing something. I picked the dual Rockets mainly because of space/location constraints and also because I needed a good musical subwoofer to support my M22’s (really the Rocket is an excellent woofer rather than a subwoofer). The trade-off was I don’t have the full impact for some movie scenes.
Unless you place a much higher priority on space minimization and accurate bass response (musicality) like I did, I’d go for the bigger subwoofer. You could always get a second one later if it doesn’t fill the room enough, and there are separate component equalizers to handle room mode problems. I didn’t go this route again for space minimization of equipment.
I will say I do like that dual front mounted subwoofers seem to make higher frequency crossover points much less noticeable, but your H&K reciever may not have the bass mangement constraints of my Denon.
