IMHO, a Harbeth will mate better sonically with a REL. I will even suggest you get two RELs (one for each channel) simply because the H30 does not go down low enough for you to be unable to detect the sub. If you can only get one sub try placing it in the centre between the speakers a few feet from the front wall but behind the speakers (place until it seemless i.e does not draw attention to itself). I am suggesting this only because you don't want the sub to give away its location. If this does not bother you then by all means place them on either side of the speakers.
Do remember that a sub will change how the Harbeth sounds so be gentle with volume. Begin by setting the XO point at around 60Hz and you should be OK.
As for the Velodyne, you may find the built in software useful if you have space constraints. If your speakers are really a good distance from the walls e.g Cardas placement method or 1/3 placement rule, there is very little use for the software.
