I've grown to love how Yes could make music that dense without it feeling "busy" or "cluttered". Props to their producers and sound engineers for that as well, I'm sure!
A lot of their 70s stuff was produced by Eddie Offord (plus the band, of course) who also produced ELP. I'm not 100% sure about Relayer (edit: it was Offord). The 5.1 was mixed by Steve Wilson who does a great job of 5.1 mixes but always seems to flatten stereo mixes. The circa 2003 remixes by Rhino are better and they come in nice slipcases.