Pat, the 3.3 bettered the 2.3 in every area. The 2.3 is not on the same playing field. The 3.3 is more open, has better highs, has a better midrange and MUCH better bass. The 2.5 is a better speaker than the 2.3. It betters the 2.3 in transparency, midrange definition and bass.
The reason I didn't get more specific is there is actually not a lot there to get specific with. It is a apple vs a grape comparison.
The 3.3 will play much, much louder.
You can go to an old Stereophile (look at their web sight) and read the review on the 3.3. It was a "B" rated speaker. The 2.3 was a poor "D" rated speaker at best. So, think of any audiophile term and the 3.3 betters it!
The 2.3 was one of NHT's initial products. They got a lot better with time.
Yes, I have owned all 3. I still have the 3.3's and the 2.5's in a home theater application.
The only area where the 2.3 betters the 3.3 is it doesn't require as much power to operate.