Bill -
I have a 5.1 SongTower home theater (2 pairs ST and one SongCenter)powered by an AVA 240/3 and AVA 440 (both Double). If you absolutely need it, I can provide a picture of the back of the 240/3 later this evening if you like. Quite simply, it looks like the back of the 440 with one exception, another set of binding posts on the rear right side, adjacent to the heat sink. So you have two on the bottom, and one on the side to drive your 3 channels.
I also use the 2100. The way I have it set up, I use the 240/3 to run the Center and the two rears, and the 440 for the front channels. The reason is, if I'm just running 2 channel music, I don't have an extra channel of power going if I had put the entire front + center on the 240/3. This works quite well, in fact, you could get an Ultravalve and use it for the fronts, and the 240/3 for the center/rears on your setup. The only downside is I'm not sure how often you'd have to buy tubes running so many hours in that type of setup, but tubes can be cheap.
For no muss, no fuss, all solid state is the way to go. I have no complaints about the sound quality of the 240 and 440 on my SongTowers. Also note that there is no 12v trigger, just the rocker switch. I'm using an Outlaw 990 as my processor if thats of any use to you. If you have any further questions, let me know.