Rel makes fine subwoofers, no doubt. They also look good, but are relatively expensive.
For a while their unique selling point was that they connected at speaker level, and without a high pass filter for the main speakers. Hence their typical low pass crossover point had to be and was relatively low, with as little impact on the main speakers as possible.. These days, many other subs can do the same.
To be honest the sound quality of a sub is largely determined by the room. 'Speed' for example is an acoustic property and not as some seem to think a driver issue. Just look at REW waterfall graphs. Room modes are a nasty problem, only really solved by the combination of multiple subs and room eq.