There is that reassuring always-there low-mids and bass response that I associate with a Rick. It must have sounded great in concert for it to be captured on a primitive tape recorder like that.
I don't like Greg's Jazz bass tone.
There are parts of the Mar Y Sol recording where it is so ice-picky that it is unlistenable, in my opinion. I've seen a clip of ELP performing Tarkus in Japan during this period, and Greg is strumming full chords on the bass. With that ultra treble tone it just sounds like noise, especially as the other end of the tone, curtsey of folded-horn cabs, is often lost even on official live recordings.
In fact I've heard too many ELP recordings where Greg is playing bass out of tune or getting lost during Pictures at an Exhibition. I do sometimes wonder if he was the weak link in ELP? As soulful and full as his voice was, he still pushed it beyond its limits onstage. I've also seen the footage of Keith basically teaching Greg parts of Karn Evil note for note. Was he basically just tasked with playing Keith's left hand parts?