You have to double the amount of power for every 3db increase in volume. 3db is a common click on the volume knob for many preamps. This means to get another 'click' worth of volume you have to go from say ...10 watts to 20 watts, 20 watts to 40 watts and so on. This also holds true at really high power levels such as moving from 500 watts to a 1,000 watts. As you can see, the law of 'diminishing returns' is at work. It may be impressive to see that 300lb,
1,000 watt amp in a system, but it is likely to not make any difference at all versus a 50 watt amp if the speakers are at all efficient. Some of the best systems I have ever heard have had amps less than 10 watts.
With regard to the choice of ARC amps, I have personally used both tube and solid state ARC amps in the past with very good results. I have never heard a bad ARC amp, although it is possible, I suppose, if the tubes are shot or the equipment piece has had some rough treatment. I buy primarily used equipment, but am picky about the visual condition of the item. Typically if it looks mint, there's a good chance that will sound good as well. One really nice thing about ARC is that they try to keep a stock of replacement parts for every product they have ever brought to market. Lots of people send their gear back to ARC for repairs or upgrades. Vintage ARC gear can sound pretty amazing in the right environments. An example is a system I had 20+ years ago using ARC M100 tube mono(s) for the treble panels of Infinty RS1B(s) and ARC D200(s) solid state for the bass towers. Lots of good memories.
Hope this helps.
