Dear jfrmusic, this needs to be approached scientifically if you hope to achieve proper integration of subs into your system. You have some really nice kit and it's worth doing what is necessary to hear what it's capable of.
The only way to do this without endless experimentation is to embrace measurement so please download REW (free) and buy a suitable mic. for $100 which will remove the guesswork. There has been some good advice here in certain repects but most of it is all over the place and confusing. Best is to go about things methodically.
Any sub like REL and its ilk are unsuitable for 1 reason: they do not have variable phase. It's also easier and better to use sealed subs which your D108 is and has a natural 12dB/octave roll-off so plug the ports on your speakers which force them to now behave as a sealed box (a good thing) which decays at 12dB/octave and so match the D108. I suggest at least 2 subs which is neccessary to truly smooth out the performance of speakers, extra bass sources (subs) and the huge influence of your room's acoustics. This brings into the picture the importance of plugging the Harbeth's ports. You have effectively removed 2 additional bass sources that resonate at only 1 specific frequency which by definition can not be tuned and will complicate set up. Point: AI has misled you claiming a 12dB slope. Your REL has a passive radiator which rolls off slightly steeper than a ported design which is 24dB/octve.
Your D108 partnered with a sealed sub like the great value SVS SB 1000 PRO with numerous controls will allow you to achieve the industry standard of +/- 3dB across the spectrum. Without the ability to see the frequency response continually change as you adjust the controls you are left guessing. REW will also display if you have sufficient absorption or perhaps too much. As I mentioned it's a scientific approach which has been well documented. It works.
A further bonus is the ability to place the subs where you would like them to be which is a function of continuously variable phase. Ignore the advice in posts above to place them behind or in front or whatever. I do recommend starting with 1 sub in a corner, any corner, and the other in the middle of the opposite wall.
I state with confidence that you will not have a smeared image nor loss of air just the opposite. By using measurement and subs and some absorption you will have effectively removed the damaging effects of your room's acoustics and will hear the acoustics of the event. The audiophools holy grail is it not? 