Which sub-help please

The Velodynes with the room EQ device are well-reguarded, but a smoothly EQ'd bass curve doesn't guaranty a seemless blend with the mains. This blend is paramount. Since you know the REL, and the Velodyne EQ unit is available as a stand-alone item, I would stick with the REL, possibly adding the bass EQ unit if you feel it would make an improvement. Just my $0.02.

I would go with the Velodyne DD series. They are better than the SPL models.

I'll second the recommendation of the Velodyne SMS-1 stand alone DRC unit (or similar, like the SVS/Audyssey) with any decent sub of your choice. DRC doesn't guarantee a seamless match, but I can assure you it improves your odds by about an order of magnitude.

Marty

PS - The 18" Velo is a monster with enormous clean sub bass output capability. IIRC, the REL 150 was an 8" model (am I mistaken?) which probably has zero sub bass output capability, but - like many RELs - is excellent down to about 50hz or so. You might look at SVS or Rythmik (my choice) which fall somewhere in-between, Rythmik leaning slightly to REL and SVS slightly to Velo. Either one is probably a good choice and definitely a good value.

BTW all products recommended here are available on a money back in-home trial basis. So, trial away.....

The Vandersteen 2WQ is what I think you are looking.
whether you are using lower powered Tube amps or any power amp the Vandy 2WQ and also the rel gets its input from speaker level.
This offers us better matching or blend from Main speakers with Sub because the main amps character is preserved.
Where the Vandersteen design takes this a step further is by a unique high pass which allows for optimized or best turnover point. This not only improves dynamics but dramatically lowers distortion increases clarity and transparency with the whole system.