Hey Gordon,
There are several ways to modify your SD2000.
1. Increase Voltage in. Your SD will run all day at 8 volts without overheating. You can buy Lithium Polymer Batteries for Radio Controlled Trucks that are 7.6Volts and weigh 95 grams. MUCH lighter than the 4 pound lead acid monster that comes with it.
2. There is a mod that adds two freqs to your SD, and makes it more sensitive.
www.minelabmods.com
3. Adding a speaker and/or volume control gets rid of another SD weakness (low volume).
Depending on how much work you want to do (or pay someone else to do), you can have an SD2000 that performs as well as or better than an SD2200.
I know because I have done all of these to my SD2000, and I am MORE than happy with what I have.
Best of luck,
Mike