Hi Belanger88.
To answer your question its hard to replicate the exact behaviour of an analog circuit designed for a die (the piece of silicon in side a part like an op-amp) with discrete parts for several reasons (I'm sure there's more but these are the ones that spring to mind):
1) as already said the geometry of transistors on a die can be very carefully controlled to be within tolerances. geometry is a fancy way of saying the size of the various bits of a transistor such as the gate or channel. these sizes have an effect on the amount of current the transistor can handle and the capacitance it has at its terminals (which in turn affects its bandwidth)
2) the doping of the transistors on a die can be very accurately controlled. doping is the amount of certain chemicals added (like doping in cycling

) which has a huge effect on the gain of the transistor and its various performance curves. Especially in op-amps the matching of gain between 2 or more transistors is important to the performance including how much distortion it creates at certain frequencies
3) on a small die (such as an op-amp) the temperature is about the same everywhere so all the transistors are at the same point on their gain and threshold curves
4) even the transistor inter-connecting wire lengths which affect resistance and capacitance, and so frequency response of the circuit, can be accurately (within tolerances) controlled
5) the package - that's the plastic lump with legs which the silicon chip is inside - also affects the audio performance since it adds some inductance on the IC's leads
Of course analog chip designers try to design out many of the variances so each chip performs the same as the next in production, but often the way to design out these effects is to use silicon manufacturing process' ability to match transistors precisely.
All of those things (except maybe #4) are almost impossible to do nearly as accurately with discrete parts.
That's not to say there's no point in trying to build an analog IC from discretes but it will not be able to have the exact same characteristics over temperature and supply voltage as an LM308 or a CA3080 or whatever. When you get to the classic synth chips (like the Curtis chips) the characteristics are even more bound up with the matching of on-die transistors and chip layout.
Let us know how you get on with this as I'm sure you'll create something really sonically interesting.