It's hard to speculate as to what will work in any given space, although Bob's point is a good one:
Smaller rooms are generally tougher all around.
You tried the full range EQ and determined that it was "clearly worse", so we now know that this approach doesn't work for you. You could try my approach ("bass-only" DRC), but I'm not sure about the best way to implement it for you. I don't think that you can use the internal Audyssey circuit in this fashion - you'd need to go with external hardware, like an SMS-1 between the sub out on your HTR and the subwoofer's input. This approach seems kind of duplicative, but it might work.
OTOH, you could simply go with dual subs and experiment with placement until you find the best locations and leave it at that.
As to driver size vs speed. There's some disagreement, but a lot of the knowledgeable writers point out that large drivers in subs are usually operating WELL below the frequency at which they'd exhibit any problems. IOW a small driver producing 60hz, isn't "faster" producing 60hz than a large driver producing 60hz.
I've speculated that some people might be using "fast" to describe a highly damped sub, since damping is related to the system's momentum/inertia (which, it appears to me, is the issue behind the objection to large "slow" drivers in subwoofers). I'm speculating here about people's issues, so please be aware - this is nothing more than speculation.
OTOH, highly damped systems actually do produce "tighter" bass (no speculation here). Just note that this characteristic is more a function of cabinet size and type relative to its given driver, rather than the size of the driver used. A well designed, giant driver can be highly damped in the right box.
Bottom line: IMHO, if you want a "fast" sub, buy a highly damped sub and driver size isn't particularly relevant. Again IMHO. A highly damped 15" sub will sound "faster" than an underdamped 8" sub. Once more, IMHO. Please understand that many folks here will disagree with these statements.
As to mixing driver sizes, I have no personal experience, nor has anyone shared such with me, so I can't really offer advice.
Good Luck.
Marty