wow, i was just reading about this last night. PBG is the one to talk to but i will try and make a stab at it, as i am understanding it.
SVT's use what is called a Baxandall (Bauxandual correct spelling i think?) preamp circuit. basically the circuit was originally desinged to use no mid preamp pot. it simply wasn't designed that way. this stabilizes the circuit by leaving a mid-tone adjustment to the treble or the bass circuits but actually does not move the midtones a lot on a graph if one were to put it on an ociliscope (sp?).
well, Ampeg did it = put a midpot in.
one thing a person has to realize is where it is put in and why. again basically 1) it is hard to put a wide sweep midpot in because of the initial design and 2) where one puts a tone circuit in, affects the overall tone a lot. i'll explain below:
there stages of gain to a preamp be it one tube or more as there are two sides or more to a tube. if one puts the tone circuit in after the first gain stage then it tends to drive the distortion higher as it amplifies or more correctly modifies the first signal and lets it go to the rest of the amp to be amplified.
if one puts the tone circuit in after the secondstage, either after the second half of the tube or after another tube depending on the design, then one is modifying the signal after it is already amplified hence you get a cleaner signal to send to the power section of the amp. this one fact is prolly the biggest difference in a nice bass amp and a guitar amp as i understand it. along with power because of the inherent power needed to drive low frequencies cleanly.
Ampeg in the SVT design used a Baxandall circuit as it is a very good design and they put it in after the first gain stage but before the second gain stage was complete if that makes sense. they put it between the second stages of the second tube. they then let the Bax circuit do its work (watch the magic crystal and be quiet so it can do its work!) and then they put a midpot in after the second stage tube and before the cathode driver (another magic thingy) almost bypassing what one might think of as a third stage gain circuit (a third tube in the tone circuit) again before the cathode driver. in effect they tried and to a good part successfully so, added the midpot to where there was none meant to be and it worked to a degeree. prolly not as well as if they had put it in the first part of the circuit but they used it to tie two channels together as well.
Fender did a different design by putting it all after the first stage (again basically a Bax circuit) with some mods on the treble end and adding a midpot there while they were doing it. not a lot of adjustment either but some as it basically raised the whole curve and not just altered the mid range. Fender designed their tone circuit to reflect a mid scoop as Leo and his engineers thought that mids in the pickups of the day were too midrangy to the point of making too much distortion. they probably were to some extent. we look at it as "vintage sound" now.
Fender never did figure out this placement or at least they didn't practice it a lot (they later got into channel switching = more magic keeping their guitard distortion!) and Ampeg married apples and oranges. PBG will tell me if i am wrong, but i am almost positive i am correct in this and i think this should answer some of your question.
it is built in. a Fender is not a Ampeg is not a Sunn. a Marshall is a Fender knock off to some (major) degree.
i enjoy apples, oranges and watermelons although not at the same time unless they are in a fruit (think boutique amps with everyones twist on things mentioned above or Uwe's analogy to the nature of a lion posted elsewhere) salad.
