Caps store charge/voltage.
I thought you knew...
Please read the "Basic safety rules" by fellow forum member Gyraf on his website (a bit down the page): http://www.gyraf.dk/gy_pd/g9/g9pd.htm
Now read your 1951 schematic again to check what voltages to expect in the Behringer unit and check the caps on the PCB against the schematic.
Caps store charge. And a high-voltage accidentally discharging into hot solder can be dangerous. So, in general, before you fire up your soldering iron to desolder a cap, make sure it doesn't hold any charge. Easy to find out with a multimeter. If it does, discharged it (using your multimeter).
IC voltage: The answers to your questions are in the datasheet. Please read it again -- carefully. IC datasheets quote minimum and maximum supply voltages. If too high, the IC gets fried.
Slew rate (Flankensteilheit) etc etc you can read about in wikipedia, if you are interested.
You might also want to read some of the META threads of this forum, such as this: META - Useful Information for Newbie DIYers (and others!):
http://www.groupdiy.com/index.php?topic=349.msg4101#msg4101
Some really great info in the many META threads. Pretty sure that once you start reading them you'll find the answers to many of your questions (asked and unasked) and will come up with many new ones. Welcome to the world of DIY...