The thing that has always struck me about these "echo machine" discussions is that the man himself seems to view all this with some amusement & bewilderment that people are going to such extreme lengths to emulate that "sound" that he & the guys took for granted in 1960/1/2 when recording.
They really did just plug the guitar into the echo machine into the amp's they were using & the studio mic'd up the amps then recorded what they heard at the sound desk (with all the studio effects/enhancements that were in vogue at that time)
I really don't think the echo is the problem it's all the other stuff like pre-amps/power amps/connections/mic's/wow flutter/compression/studio added stuff..............the list is endless down as far to was it raining/ambient temperature.....all these things have an effect on the recorded sound to some degree or another.
To try & replicate all that in one "box" is always going to be impossible to get 100%
Sadly digital processing is the only chance there is that all this information can be accessed & processed at anything like a "reasonable" development/manufacturing/cost-performance ratio.
I've heard people play (both as a band & solo with bt's) through an Alesis (GT) straight into an AC30 & sound just wonderful & that to me would be my "Holy Grail" if there is such a thing. However I sold my AC15 ( and MStomp)in favour of a Fender Blues Junior/Etap2hw so I'm a bit of a lost cause
Dick.