This has been an interesting thread and I thought I'd just throw in a few observations.
First, we have had three owners upgrade from SongTowers to HT2-TL's so far and all three regard this as a significant upgrade. So I think there is more at play here than simply the cost differential.
From comments I have received and from my own personal listening, I would describe the major difference between these models as the ability to scale deeper bass.
As an example, all of our speakers do quite a good disappearing act and provide a reasonable illusion that the performers are actually in the room with you. But with most models, that illusion begins to fall slightly short in the bass area (as would be expected).
The HT3's scale bass appropriately for the performance and extend the illusion through the bass regions. Play a double bass on the HT3's and it is as if it is being played in the room. With the SongTowers, the bass is mostly there, but the illusion of a performer playing live in front of you is not as realistic. At these lower frequencies, the bass does not quite scale appropriately to create the illusion.
In that regard, the HT2-TL's will get you closer than the SongTowers. When it comes to low bass, there is no substitute for displacement. And the HT2-TL's simply have more displacement than the SongTowers.
There may be slightly more detail with the W18's in the HT2-TL's. But when people describe the difference to me, they do not generally refer to midrange detail. Most of their comments are centered around bass response and they feel the HT2-TL's are superior in this regard (which would only make sense due to the increased displacement of the drivers).
For certain types of music, it is not an issue. Classical music, for example, does not generally have a great deal of low frequency content (this is not a universal statement, but a general one). In that case, the difference between these speakers may not be all that great. But with other types of music, the bass extension and scale of the HT2-TL's is of great benefit.
That said, if either of these speakers were the only ones I had, I would not be at all unhappy. As most people know, I regard midrange response as being by far the most important. That is where 80% of the information is contained. And I think that all of our speakers perform very well in this regard (or we wouldn't have introduced them). As you move up in cost, the differences between models will be found mainly in the transparency of the top end and the extension of the bass response. That's about as easy as I can explain it and hope that helps.
- Jim