It's voodoo, really...while I can understand stereo field depth, how Brian Wilson and Chuck Britz got that depth in a MONO mix still mystifies me...along with a lot of other people. "Pet Sounds" is so full of audio mysteries, one could (and many HAVE) spend one's entire doctoral dissertation in audio engineering studying it and breaking down what can be gleaned from it. Some of what I hear, though, seems to involve the amount of reverb added to different layers along with some very hi-tweak level settings. But there's a problem there, in that what they used for reverb weren't clear. Mark Linett thinks they used plates, but from my experiences with my own Ecoplate II, I'm more inclined to think that we're hearing a combination of those on some isolated parts, and the rest is piped thru United Western's chamber. Nevertheless, the album only uses tape slap and reverb for processing, and there's some indication that they actually printed the reverb on the original takes in many cases, not opting to add all of it in mixing.
The biggest problem, though, lies with Brian himself...this album marked the beginning of his acid-fueled mental decline, something which would go full-blown when work on "Smile" was under way. And since Chuck Britz died in 2000, the actual details of HOW this was mixed are rather lost now. It's known, though, that this was mainly done on 4-track machines, not unlike what The Beatles would also use on "Sgt. Pepper", and "Pet Sounds" may well have pushed that decision by them and George Martin. So, yeah...this album remains an amazing audio engineering mystery in a number of aspects; reading how Mark Linett had to re-piece together all of the master reels for the stereo mix in the reissue's liner notes just sounds like utter madness! Nevertheless, the album is a frickin' monument...it's arguable that ALL prog started from it, given its popularity in the UK and its similarities to the first prog albums that would start to appear in subsequent years. But in the USA, it was largely dismissed when it came out...it's only been in the past 20 years or so that the importance of "Pet Sounds" has become lots clearer. Frankly, I'd put it right up there with Stockhausen's "Kontakte" as a body of work that requires careful (and rewarding!) study.