The input impedence of the speaker-level inputs to the REL is way way above the speaker's impedence, and so high that current delivery is just not an issue, and extraordinarily easy for your amp to deal with. Therefore the cable requirements are quite different from speaker cables. It therefore also has little impact on what you hear out of your main speakers. The claimed benefit is that you get some of the flavour of your amp/speaker interaction input to the REL - providing a more seamless blend. In practise I find both the speaker level and line level to work well. The biggest issue with RELs is that they are designed to augment your main speakers, not share the load with your main speakers (through an electronic cross-over). I find augmentation to be the only acceptable way to use a sub-woofer, else the additional cross-over spoils the sound through the main speakers.
Natural sound (REL Subwoofers)
I am looking for a natural sounding subwoofer, and I hear that REL makes some of the best in the business. I am interested in the REl Stadium and Stentor. Has anyone had the opportunity to try these subs or make comparisons to similarly high-priced subs? I have heard less expensive subs like Sunfire and Velodyne and they seem too boomy, and therefore not worth the money...I am looking for something that will add ambient "space" to my music. I have heard the Snell 18" subs that go with the reference "A" speakers ...very nice sound but too big. Others that I am considering are Audio Physic, and Talon. Please tell me about your experiences, preferences, and biases. I have an aleph 3, proac 2.5s and plan on running them straight from a resolution audio cd55. Thanks