Line level or speaker level, scientific reasons? ?

Running in parellel is the cleanest way but not always the sonically best. My beef is that running in parellel will not always blend well. You can set the sub volume at a certain level and have it match great but once you change pre-amp volumes, many times your blend won't work as well simply due to efficiency differences and a difference in amps between the sub and main amp. Yes, you can have a remote volume for the REL but who wants to keep messing with sub volume?

I have to agree with Elevick that it has been a little difficult to find a perfect blend. It seems at some frequencies it's perfect, yet at others it is noticeably that there is a sub providing extra juice. I"m going to try and mess with the crossover and phase setting to see what may help.

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.