Hello, I am a new member here and hoping this forum can help me on my first project. I am not looking to build an amp from scratch, but looking to modify an existing amp. My starting point is a Marshall 3203 Artist head. I've had this amp since I was 17 and have enjoyed it quite a bit. I have since begun using a pure tube amp and the Artist has sat in the closet for a few years. Recently I got interested in effect pedal modifications and noticed people using different opamps, diodes, and capacitors to improve the tone of effect pedals. So I wondered if the same things could apply to my Artist head? This model amp is far from a popular Marshall model, I've found very little information about it online and nothing about modifications. So the first question is, where to begin?

I have never liked the sound of the clean channel on this amp, it always sounded thin and weak. I would like to make the clean channel sound fuller and warmer.  I would also like the amp to have more dynamics overall and not sound so much like a solid state amp. Seeing as it has a single 12AX7 and two EL34 tubes I would think that a few changes in the preamp could make the amp sound more comparable to a full tube amp, such as a JCM800.

I have found the schematics online, however this raises another question as the schematics seem to show six op-amps, while I only see four op-amps inside the amp? They are all Texas Instruments MC1458P's. Is there another model chip that would help me achieve the changes I want to make in the amp, and what other changes should I make to go with them, such as different types/values of diodes and capacitors.

If anybody can offer some information or guidance on this project, or at least point me toward some good information sources it would be greatly appreciated. Thank you.