I had the same question when I was trying to decided which sub to get. Here is the conclusion that I came up with. If you plan to use your sub for music and for the LFE in home theater, the difference will be huge. If you are just going to be using it for music, the difference will not be as great. I believe all the version III's comes with the ability to switch between using REL's internal crossover for use in 2-channel audio and bypass for the LFE for movie soundtracks by a switch on the sub. With the previous version, you will have to physically disconnect the RCA's low level input when using the sub for music or disconnect the high level input for movies soundtracks. If one does not do this and both inputs are hooked up at the same time, your bass output will sound like its doubled. The version III's also come with a bit more power with an improved enclosure.
I don't really believe you can have too much subwoofer as you can control always control the output. I am using a Stadium III in a room that is 15x17x7 and am very happy with the performance. I think a lot depends on your main speakers. I am running the REL with Thiel 22's and crossing it over at 23hz, the second to lowest crossover point. I would personally prefer a Stadium III over a Stentor II.
Hope this helps you in your decision.
I don't really believe you can have too much subwoofer as you can control always control the output. I am using a Stadium III in a room that is 15x17x7 and am very happy with the performance. I think a lot depends on your main speakers. I am running the REL with Thiel 22's and crossing it over at 23hz, the second to lowest crossover point. I would personally prefer a Stadium III over a Stentor II.
Hope this helps you in your decision.