I've found that it's not a big deal whether the powered sub is driven with line level or speaker level. It's not a big load on either. The argument can be made that the line level setup has less distortion since the amp is bypassed and so is whatever distortion the amp has is also bypassed. The argument can also be made that the speaker level will give sound that is more coherent with the main speaker.
The main issue is whether to use the sub cross-over to cut-off the low frequencies to the mains. I used to use this setup. It was a nice idea that the cross-over frequency can be set on the back of the sub and both it and the mains would be crossed over correctly. Advice from previous Audiogon threads was that most sub-woofer x-overs were not very good quality. I believe that Rel is an exception here. I changed to driving the mains directly from the amp and the sound is much better. The idea is to not mess up the main speakers using a low quality cross-over. The disadvantage is that I have to set the sub crossover point to blend in with the natural roll-off of the mains. The sub is just the filler for the last bottom octave or two.
The main issue is whether to use the sub cross-over to cut-off the low frequencies to the mains. I used to use this setup. It was a nice idea that the cross-over frequency can be set on the back of the sub and both it and the mains would be crossed over correctly. Advice from previous Audiogon threads was that most sub-woofer x-overs were not very good quality. I believe that Rel is an exception here. I changed to driving the mains directly from the amp and the sound is much better. The idea is to not mess up the main speakers using a low quality cross-over. The disadvantage is that I have to set the sub crossover point to blend in with the natural roll-off of the mains. The sub is just the filler for the last bottom octave or two.