TS pedal don't so much boost the mids as they do cut the treble and bass, this may seem like a pedantic difference but i do consider it to be a difference none the less. People refer to it as the midhump, which i guess could be diescribed as a passive influence rather than the active boosting of a graphic eq.
In real terms of sound, it think an active mid boost serves to give you more snarl/crunch and grit in the midrange. Active mid boosts that do it well will give you a form of seperation and clarity in the mid range (even under high gain conditions) IMO.
TS's in their earlier days were (and still are used, esp. by the country mob) to add to the mids of amps voiced with big midrange (in relative terms, think the Stereotypical Fender and Boogie sounds) scoops. Now there is debate as to what effect mids introduced from an eq external to the tone stack has on the amp (is it the same as dialling in more mids from the tonestack, is the result different?), and i won't get into what i think is going on more than "briefly", because i'm inarticulate about it. Some of the guys here have some well described and easy to understand views which hopefully they'll contribute. It is felt that the tighten up the bottom end of your standard rock amps such as marshall, 5150's, which i agree with. however i feel the trade off is flub in the midrange, where, to me a guitar lives. In a live mix, i feel the bottom end is less important as the bass player lives there. I think the main thing is cutting thru with the lower and upper midrange.
On the delay thing, my take on it is that if you're using any kind of pre-amp (internal to the amp) gain in a high gain amp, then you want your delay in the loop, if its all dirt boxes with minimal amp preamp gain then go in the front end.
IMHO effecting the distorted signal is preferable to distorting the effected signal. there is a difference, and its up to you to decide which you prefer.
I have less choice than most because a) i prefer "effecting the distorted signal", b) my loop is line level only and c) i prefer analog delay sounds over tape or digital. Fortunately Strymon, Eventide and TC exist (especially Strymon). So i use a Strymon Brigadier which gives me exactly what i want, with the added bonus that the dry signal remains analog and only the wet signal is digital (and it has a very effective mix knob!).
