Hi Jaffa,
That is a decent size room that you have. I would have to believe that a speaker that goes down to about 28-30hz, will suffice.
You could certainly get "too big" and that won't be good either. The goal should be to get the right sized speaker for your room.
You mentioned that you want a more percussive attack. This is usually in the Bass region 60Hz-250Hz, not the Sub-Bass region 20Hz-60Hz. I feel that your Monitor and Subwoofer won't solve this problem, because your monitor isn't producing enough punch in the bass region and your subwoofer can't reach high enough into the bass region either. I actually prefer the sound of a Floor-Standing speaker with a Subwoofer, than a Monitor with a Subwoofer.
I believe that you need a Floor-Standing Speaker with at least 3 drivers (Woofer, Midrange, and Tweeter). This way the woofer can handle the bass region and the midrange can handle it's region without being pushed harder to perform in the bass region. A Floor-Stander with 4 drivers would also work.., but then you may have two smaller, faster woofers, one midrange, and one tweeter. Since you like a fast percussive sound, I would think that 2 smaller 170mm woofers, instead of one larger 250mm woofer, may be more to your liking.
Although, I have never listened to the PMC PB1i, I did check them out online and I think you're on the right track with that type of speaker design. The rest is up to you my friend...
Good Luck,
Rich