How much $ ?

There's an early version of the Series 400, and a later version. The later version has fuzz, distortion, and blend knobs on the left side, with a slope control under the volume knob on the right side. This version has an extra set of input jacks in the middle; with these jacks, and the footswitch, you've got the capability of channel switching and combining in series or parallel, plus fuzz and distortion in/out on the footswitch. These are more desirable than the earlier versions, so $125 or so is a decent price for the later version if the amp is in good condition. Power output (for both versions) is around 140 watts into 4 ohms, and 210 watts into 2 ohms. The earlier version Series 400 would be worth less, maybe $100 or so...

Don't confuse this with the earlier Series 300 which has less power and a very basic set of controls...