1. Is the high pass filter of good quality in the Velodyne?
It will be orders of magnitude better than the speakers you'll be connecting to it, but it'll also be the first device to get blamed for a poor setup. You can always buy an NHT X2 filter if you believe the internal filter is poor.
Even as good as Velodyne subs are, I wouldn't mate a 10" model with Harbeth 30s. The 12" would be as small as I would consider and I'd take the 15" if budget can be managed.
2. Can the Velodyne be hooked up in the same way as the REL by running speaker level signal from the amp to the sub?
Yes. All direct drive models have speaker level connections.
3. Of the options offered by the Velodyne, which way would you recommend connecting for blending well with Harbeth M30's which extend to about 50Hz?
High-pass no question. Regardless of the M30s specs, I doubt you'll get 50Hz out of them unless you get them close to a wall, which you do not want to do. The big downside to the internal high-pass filter is that it's a first order design. So, an audible portion of a 40Hz signal will still be sent to the M30s. Of course, if you go speaker level connections, then all of a 40Hz signal will be sent to the M30s.
The critical aspect of setup is placement and level matching. The Velodyne's PEQ will help with any placement issues, but level matching has to be done with test tones and an SPL meter.