Guitar Toad- Good question, I tried to roll too many thoughts into one sentence-
First I completely agree with Zhangliqun's post that the middle pup is the spice that gets quack and that quack is one of most importand strat sounds and that the middle pup by itself tends to be rather colorless- Of course these are rather broad genrealizations, but overall I think he's dead on-
As far as 70's strat, there are really 2 things at work- First of all most of the strats weren't 5 way back in the dark ages, and alhtough everyone knew the ballance trick, it's wasn't easy live-
But IMHO there was another thing going on that affected the recording side- It was far more likely that someone would take the time to get quack sounds in the studio, but in a complex mix, much of this color was lost to limited bandwidth and noise floor. Althouhg there are exceptions (Gilmore and Knophler are two of my favorites), most of the strat sounds , especially rhythem , sound either bridge like (lots of R&B stuff ) or slightly brighter than neck. My guess is that a lot of the slightly brighter than neck sounds are 4 position but lost the upper range in a complex mix.
As a result, when coverign the average strat sound from the 70's I find quack is too thin to reproduce the feeling and in the past found brdige or neck pup best most of the time-
But when I switched to twin coils I found that the middle pup became extremely useful for the first time outside of quack- By using a little of the 2nd coil to fatten the middle pup I found I could get a brigther than neck, but less bright than bridge sound that works really well for that 'strat in the back' kind of sound-
Still don't use it more than 20% of the time-
This making more sense?