Looks like we have similar tastes in speakers, Mystery. My collection includes TF-600, Polk M5 Jr+, and ADS L520. I paid $150 for the oak TF-600s from a CL seller in the Denver/Boulder area. They were in very good condition except for a tear in the back of one of the socks. If the TF-400s are series II, they are pretty comparable to the TF-600s and should sell for ~$100 in a local sale if in good condition. Personally, I think DCM TFs are undervalued in the marketplace, probably because they are costly to ship, fussy to repair, and out of step with current market preferences for small bookshelf/satellite speakers that disappear in a room.
In comparison to my Polks and ADSes (both of which are a step below your models), my TF-600s are a cut above in terms of imaging and soundstage, which is their specialty. With the transmission line enclosure, they also go a bit lower (although I'd bet the ADS L-620 beats the TF-400 in this department). Despite all the tweeters in the TF-600s, they are not as bright or detailed as the others. I'd characterize them as somewhat veiled sounding, but very expansive and full. They're great all-rounders that work well with a wide variety of gear and styles of music.
The little Polks sound great and make me think it would be worth pursuing one of the larger Monitor or SDA models. Mine have SL-2000 tweeters, which are very bright and can become a bit fatiguing in long listening sessions. On the plus side, they image almost as well as the DCMs and sound great with a subwoofer to help out the low end. I like them most for rock and metal.
The ADSes are just plain sweet. The highs are detailed without being harsh, and the bass is surprisingly good for just an 8" acoustic suspension woofer. They're only average in the imaging department, but they do everything else very well and are an absolute pleasure to listen to. They are my favorite speakers for listening at low-medium volume levels.
"Better" is in the ear of the beholder, but I'd say the TF-400s are better than the Polk 5Bs in most respects. Given their original prices, however, that's not really a fair fight. It would be a much tougher call between the TF-400s and ADL L-620s. Even though I am a soundstage junkie, I would have a difficult time deciding whether to keep my TF-600s or ADS L-520s if forced to choose one. The beautiful ADS cabinets and smaller size might end up breaking that tie.
If you can get the TF-400s (especially series IIs) for a good price and have the space for them, I'd recommend going for it. They are different enough from your Polks and ADSes (and the other speakers in your sig) that they won't seem redundant. I expect you'll find them to be keepers until you are ready to upgrade from this general class of speakers. Good luck and let us know what you think if you get them.