many info can be found on Hp, Tektronix, Lecroy etc service manuals. But may be you have already took a look there.... Many interesting things on this theme was published on Tek Journal too, I haven't it but you are on internet and all is possible....
I'm with syzzler, service manuals are great. What part are you interested in? Discrete (without custom ICs) 1Mohm input stages are tough. The Tektronix 454/454A 150MHz portable scope schematics may be of interest. I think this was near the end of the line without custom ICs. Vertical Amplifier Circuits (Tektronix Circuit Concepts series, 1969) is a good reference but getting harder to come by. Good Luck