Main set up, Snow Leopard "master password" problem ... •••  ????

Are you referring to your user account's Admin password or are you referring to the Firmware Master Password? For the former:

Forgot Your Account Password

For Snow Leopard and earlier with installer DVD

Mac OS X 10.6- If you forget your administrator password,

OS X- Changing or resetting an account password (Snow Leopard and earlier).

For Snow Leopard and earlier without installer DVD

How to reset your Mac OS X password without an installer disc | MacYourself

Reset OS X Password Without an OS X CD — Tech News and Analysis

How To Create A New Administrator Account - Hack Mac

Gotcha! That would be your FileVault password. Did you create a recovery key to register with Apple for just this particular issue? There is a bypass method for startup: How to bypass FileVault when you restart your Mac | MacIssues. I don't think that helps with getting your original password. Finally, see if this helps: Change or reset the password of an OS X user account - Apple Support.

If all fails, then you are out of luck and will have to reformat the disk without any recovery possible.

The disk need not be replaced if it is still working. You can fully reformat the disk even if the disk is encrypted. The first link tells you how to startup without a password. You can then erase the OS X volume and reinstall OS X. But if you are going to replace the disk, then the FileVault encryption is no longer relevant unless you want files from that disk.

A "disk" is the same as a "drive." Also, sometimes called a Hard Disk Drive or HDD. That is the main internal disk drive. Here's what you can do. You will need to use the Snow Leopard installer DVD:

Snow Leopard Erase and Install

1. Insert Snow Leopard DVD into the optical drive and restart the computer.
2. Immediately upon hearing the chime hold down the C key.
3. Release the key when the Apple logo appears and wait for the loading to finish.
4. After the installer loads select your language and click on the Continue button. When the menu bar appears select Disk Utility from the Utilities menu. After Disk Utility loads select the out-dented disk entry from the side list (mfgr.'s ID and drive size.) Click on the Partition tab in the Disk Utility’s right window. Set the number of partitions to (1) from the drop down menu. Click on Options button and select GUID, click on OK, set the format type to MacOS Extended, Journaled. Finally, click on the Apply button.
5. After formatting has finished quit Disk Utility. Continue with the OS X installation and follow the directions.
6. When the installation has finished the computer will restart into the Setup Assistant. After you complete the Setup Assistant you will be running a fresh installation of Snow Leopard. Open Software Update and install the recommended updates.
7. If you are planning to sell or give your computer away, then do the following: After you reformat your hard drive and reinstall Snow Leopard, the computer restarts to a Welcome screen. To leave the Mac in an OOTB (Out Of The Box) state, don't continue with the setup. Instead, press Command and Q keys to shut down the computer. The Setup Assistant will guide the new owner through the setup process after he/she turns on the computer.
8. Download and install Mac OS X 10.6.8 Update Combo v1.1. if needed. It is not needed if you are selling or giving away the computer.

Now, if the computer's disk has Lion or later already installed on it, then you can try the following instead:

Install Lion Through Mavericks from Scratch

1. Restart the computer and after the chime press and hold down the COMMAND and R keys until the Apple logo appears. Wait for the Utility Menu to appear.
2. Select Disk Utility from the main menu and click on the Continue button.
3. After Disk Utility loads select the volume (this is the indented entry, usually Macintosh HD) from the side list. Click on the Erase tab in Disk Utility's main window.
4. Set the Format type to Mac OS Extended (Journaled.) Click on the Apply button
5. When the process has completed quit Disk Utility and return to the Utility Menu.
6. SelectReinstall OS X and click on the Continue button.
7. This will install the version of OS X you had installed.

[Permission to use part or all of the above has been granted by Kappy, exclusively, to theratter.]