In a 13ft long room you'll have a 43Hz resonance. You're already over speakered by the Regas as they'll have bass response that will already be excessively driving the bass resonance of the room. Why do you want to make that worse by adding a sub and driving the room even harder?
That's interesting, but I thought that adding a subwoofer wasn't all about bass response, but would also provide a higher level of dynamics.
A sub is primarily about bass extension.
Having said that the psychoacoustic effect of the increased bass extension gives an increased sense of weight and scale to the music.
When using a sub, most of the bass note harmonics are presented by the main speaker - this is why it's important to get the integration of the sub -> main speaker right in terms of phase response (i.e. group time delay) as well as frequency response.
If you have significant softening of low bass dynamics or a 'one note bass' type of issue than your problem is almost certainly the room not your speakers. A sub won't solve this problem, in fact it'll make it worse. To fix this you need to add a load of bass traps, and the only way to find out how much bass trapping you need is to measure the room response with a microphone. The one in your smartphone may give some indication, but the have very poor / uneven response below about 50Hz and your problem is at 43Hz (almost the same frequency as the issue I have in my room).
In my room the 42Hz resonance was +22dB: It required Bass traps with a surface area of 8m2 to ameliorate it. Only then could the sub I use work properly.
To improve bass dynamics you may well also need to block the reflex ports in your speaker cabinets as the sound from the port is delayed by a time equal to a half cycle at the port tuning frequency. This increases the low bass out put (and hence increases the bass extension of the speaker) but at the expense of softening the bass dynamics. This applies even more so to subs, if you primarily want to improve dynamics: First add enough bass traps, then buy a sealed sub not a ported one.