If I can butt in here...
Look for an amp that's dead quiet when it's on and you're not plugged in. Noise is a symptom of dead or dying tubes, capacitors, and resistors. Also, look for an amp with easily replacable tubes, like 5Y3, GZ34, 5U4, 6L6, 5881, EL-34, 6V6, EL-84, 12AX7, 12AU7, 12AT7, or 6SL7. Avoid amps with odd-ball tubes unless you're willing to take up the search for obscure tubes when the ones in the amp finally give up the ghost. Gibson, Fender, Magnatone, Silvertone, Marshall, Ampeg, Supro, National, and Harmony all made decent amps. Maybe you'll find a deal on a great old amp from the 60's.