Speakers with built in sub any good?

Artemus, don't think the Step will fill yr. room. I second Doug's suggestion (Thiels). In my experience, they need very careful positioning, otherwise U get too MUCH bass -- but yr room looks fine on screen for them. Also, Thiels are power guzzlers, as Doug says.
The Genesis V or 500, with a servo controlled, built-in sub, are usually expensive used (BUT, U never know...). It takes time to fine-tune them, but the rewards are very good IMHO, especially in as long a space as yours. I have them and am very happy with them. They too, like amp power.
Any way U can audition some products? To get a feel of how much better (or worse) the sound gets vs. yr present gear...

FYI, There is a pair of Genesis V's on e-bay, might be
had real reasonable!!?? Starting out at 1k.

They have awesome bass with their seperate 400 W/CH
amp and 8 8" woofers.

You might consider Vandersteen Model IV's. Sub is built in & fires to the floor, so placement is not a problem as with side firing subs. These can be had for aroud $1,000 to $1,500 for a good used pair. Also a great sounding speaker for its price range.