Chris, et al,
Yeah I have everything related to the P603 module but for a number of reasons can't release anything beyond a few pictures - email me if you'd like something.
Key to the operation of all the Cello preamps (and power amp input stages) was their development of what were effectively, op-amp modules constructed with discreet parts. They had a series of OTA modules which were VERY low noise / high speed voltage amplifiers, and a series of VF modules which were current amps configured for various ouptut drive / load requirements.
The Audio Suite used these in their modules - phono, line level and tape input modules as well as their output modules, and their later preamps incorporated subsequent versions as well. The P603 tape preamp used the same amplifying circuit as their P100 phono preamp with the exception of the (active) equalization network. The 603 had a bunch of 10 turn pots for "tweaking" the tape response at 30hz, 1kHz and 10Khz (Similar to what is in the tubed Manley tape repro electronics).
As an aside, I "grafted" the 603 circuit onto the P100 cards I have and found that some of the pots didn't do much of anything (as I also found playing with the controls on the Manley). What I've found matters most is the ablilty to control head damping (to minimize head bumps without rolling off the lows) and some method of easily varying the HF pole response (up around 10Khz). Can only do this well if you have the ability to sweep the entire audio spectrum with a tape / analyzer setup like what the ST 1500 provides. Also have tried to "recreate" Cellos op amp modules, but found (as they did) that parts like critical semiconductors get obsoleted and finding suitable replacements may be next to impossible. Cello never obsoleted any of their products and they were selling Audio Suites (first introduced in 1985) right up until they went out of business in early 2001! The "speed" of their op amps is what I believe gives them their superior sound - superior to everything else I've listened to. They get the transients right (leading / falling edges with minimal overshoot) - am working with square waves coupled to the head via a flux loop.