"Do you use the speaker switch with your second amp and the three speakers?"
Yes. And I'll let you in on a secret. I bought the Harmon Kardon (very sweet comsumer stereo amp), the Klipsch's, and the Fisher's and the speaker switches at GARAGE SALES! I don't know where you live in NC (I Love that state!), but check out your local garage and estate sales - you'd be surprised what you can find. I've bought amps, guitars, speakers, mics at these things, all for $.05 on the dollar! It's a good way to build a studio on a shoestring budget.
"should I use the speaker switch with my old amp and my JBLs and regular stereo speakers?"
Yes. Just make sure you run the switch off of the Main (usually "A") front speaker leads on the amp. The "B" or "rear" leads often have less power the the "A's". You can get the little Radio Shack switches for around $25.
Bob