For $600, you will have to prowl eBay and every music shop you can get to. Even used, you aren't likely to find a 4x12 cab and a 100 Watt tube amp. You might be able to get a transistor amp and cab for that kind of money.

One of the best-kept secrets of the guitar world is the late-60's-early-70's Traynor YB1-A. This 100 Watt tube head is a serious competitor to the Marshall 'plexi'. Prices are starting to rise as word gets around, but you should be able to get one for less than $400. The 50 Watt YB1 is just as good, just as loud, and usually less expensive.

The Ampeg V2 (60 Watts) and V4 (120 Watts) from the same time period are also good candidates. I prefer their sound (I have the VT-40 and VT-22 combo versions) and the tone controls are more flexible than anything else out there, but they cost more, and if you buy on eBay the shipping cost is going to hurt!