Thanks for the advice, everyone.
Just as an update, I went to see the man's Tamron. I tested both lenses out, and was pleasantly surprised by the lens. I didn't end up making the trade, though.
1) I was surprised to see that the DA 40 was a much brighter lens than the Tammy at 40mm. The difference in shutter speed at f2.8 was between 1/40 and 1/80!
2) The sample he has was ok-sharp, but not as sharp as my LTD. It was obvious, even fully zoomed out. Furthermore, his lens looked fairly beat up, and had a rather long piece of hair (or something) underneath the front element. In general, I was not convinced that the lens was treated well. I feel as though a better sample of this zoom might be out there, waiting for me, which will be sharp enough to make me forget about my little limited (his sample did not look at sharp as the photo above).
The difference in equivalent exposures was quite amazing to me. While the DA 40 might not be a DOF king, it went to show me that it does not have to be a dark lens. For now, I will keep the DA 40 (and might actually consider the 70, now that I have some experience with the Tammy), but the price is right for that zoom, so if I manage to find a good sample I think I will have to bring one home with me
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Thank you everyone, and Merry Christmas/Happy Holidays!!