Scott,
Yes.
Unless you use DRC, in which case the answer is "maybe".
One sub, properly placed will usually produce much smoother bass response than a full range speaker in free space. There may be other issues with this approach(integration, for one) that, for some people, impose too steep a price to pay for this benefit.
Two (or more) properly placed subs will virtually always produce much smoother bass response than a single sub. The integration issues with your main speakers may well still exist.
OTOH, properly implemented DRC will address both FR and integration. It very effectively smooths the response of a single sub at your listening position, so a single sub w/DRC may be enough to address the FR smoothing issues. And....
My Velo SMS-1 sub controller includes RTA which displays the on-axis FR at the listening position on a video monitor. This has 2 benefits. It makes placing sub(s) much easier. That is, you can use the SMS-1 just as a set-up aid to determine optimum position for your sub(s) by looking at the FR readout. Trust me, this is easier and faster that testing trial and error by ear. If you must, you can even use the RTA to "put you in the ballpark" and then further tweak by ear.
And, the bonus...
The flexible low pass x-over in the SMS-1, coupled with the RTA readout and Parametric EQ allows for a lot of flexibility in fine tuning the "hand-off" at the x-over frequency. (IMHO) This can allow for a seamless, inaudible (again, IMHO) x-over from subs to mains. It's quite a bit of work, but (yet again, IMHO) definitely worth it.
The more expensive SVS/Audyssey unit is automatic (easier), but I'm not sure that you can use it as a placement aid and then remove it. To do that, you need a visual display of the uncorrected FR and I'm not sure that this unit has that capability. Perhaps someone else out there knows for sure.
I use both 2 subs and the SMS-1 and get great results. This approach is not necessarily for everybody, but EVERYBODY (hearing impaired excepted, I suppose) will instantly hear the difference between an uncorrected sub and a DRC sub in almost any room (anechoic chamber excepted). I wouldn't live without it. YMMV.
Marty