Which REL sub, R528 or B2

Rel is till too slow. For stereo I would not want it even for free. Respons is not fast enough, same problem as Velodyne has. Wenn you use it in stereo intruments and voices are not small enough in proportion. I have done these tests many time. For me it is like 1 and 1 is 2. I use a Monitor Audio PLW-15 with Audessey Pro with my way of measurement. Now I have stralth low freq. Respons of the unit is in timing superior compared to Velodyne and Rel. Ones you heard this you can not go back anymore. It is that easy.

A few years back I had an opportunity to audition a REL Studio III, JL Audio 113, and a Velodyne DD 18.

Your description of the REL is what I heard along with the difficulty of integration. Running it through the Velodyne I was able to EQ out the bloat between 35-45hz in my room. Still, its presentation lacked the leading edge speed of the others. Using JL's onboard EQ alone I was always aware of its presences in the room.

The Velodyne completely disappeared until I turned it off. This was a huge difference from the REL. I've since replaced the DD 18 with two DD 12+s. As a working Bassist I have many raw track recordings on tape and digital files and I have never encountered any timing, coloration, or timbre issues with any of the Velodyne subs I've owned.

The ability to adjust phase, volume, and EQ presets on the fly with the Velodyne's remote control takes sub ownership into fun status.

My experience is limited to Velodyne SPL-1200R. I was not able to fully integrate it with the mains. I could feel some strange body in the system. Bass was to tough, not musical and tiring after long listening. I used hi level connection then. Maybe if I had used a better quality speaker cable ....
and reliability problem. I used to leave the sub in st-by mode. One day, after thunderstorm ... My electronic friend looked inside, no fuses, no protection, assembly quality like those from China. The authorised dealer said you have to replace whole electronics, can't replace parts. This is how it's designed. So I said to myself no more Velodyne.
The advantage of REL is it's connectivity, I'd like to use it in both, HT and stereo system.. I took a look what the guys from positive f use, mostly rel's if any. Unfortunatelly they are not available in my area for a test. I found some posts here, on audiogon saying a few persons used C1 and REL's with success hence the decision.
Thanks for your input, I'll have to rethink it....