Subwoofer Choice

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.

I would point out that regardless of the method of speaker placement the Velodyne software is still very helpful in sub placement, crossover point, level matching regardless of whichever sub you chose. Velodyne also markets this system as a stand alone and typically sells for about $400 used.

In a recent review of the JL Audio sub the reviewer used his Velodyne software to setup and provide a visual of the JL's own setup software. Rives Audio sub-PARC crossover manages these parameters in the analog domain.

Vic

Is the Velodyne software compatible with Macintosh computers, or only Windows based systems?

Also, I read somewhere that the newest version of the Velodyne software does not permit crossovers lower than 40Hz. Is this true?

Rel is generally considered to be better subs for 2 channel with its ability to blend seamless and dissapear. Velodyne makes excellent subs and the digital correction is a great feature but sub for sub I would go for Rel. I have heard a moderately priced and sized B series (forgot wich one) that was amazing. I ended up buying a Martin Logan Descent and will probably add an sms-1 someday.