the 'tightening up' feeling is just a side effect of the EQ response of the pedal. often boosters cut bass, which will make a vintage amp like a plexi or master volume sound tighter. plus they hopefully give a distortion which compliments the normal amp valve overdrive, and also a general level boost to overdrive the input valve even more.
i agree with twinfan that a TS pedal doesn't really tighten an amp. in my opinion a TS is very lead orientated. however there are quite a few TS derivatives which are more general purpose and do tighten up rhythm and so on - perhaps a Boss SD-1. i do think that TS lead sound is a very difficult one to beat though.
if you just want to tighten stuff up, you could think about an EQ pedal - it will give you more control than the single EQ control of an overdrive pedal.
