Just as many others are suggesting, I'd be looking at some other solution rather then adding ambient tweeters closer to the mids.
Get your tuning and positioning good, and an extra set of tweeters won't be necessary.
Likewise, if your tuning and positioning is bad, then an extra set of tweets won't save it - no matter where they are. You'll situation will likely resemble a dog chasing it's tail.
If you must, then I've been fairly impressed by Oz Matrix, Morel Supremo and Scanspeak tweeters.
Hertz mille I found to be too lacking in the top end.
Seas Reference I've heard a second time and this time they sounded much, much better. Whether this was a result of more running in time (or a result of them being moved into a larger area) I'm not sure, but as a set they sounded much better. As far as the tweeters they actually aren't half bad. Very innofensive - quite well ballanced overall, but a tad soft in the absolute top end. If you need something that won't scream out, they may be a good choice - however if their response varies to that of your current tweeters (which i generally will) then that will create immediate issues in your high frequency response. Best to stick to a second pair of the same tweeter if you can - or again, just stick with the one pair.
QUOTE (ar3nbe @ Nov 11 2007, 04:15 PM)

I believe a speaker can be smooth, and however lack detail. Smoothness has more to be with frequency response, a nice flatish speaker with no massive peaks or dips will tend to sound smooth ( doesn't mean flat frequency response, just no massive jumps down or up). Detail on the otherhand relates to something a little different.
I also said detail may of lacked a bit, a bit meaning a small amount. And the speakers have had a fair bit of playback on them, perhaps 80 or so hours.
Both smoothness and detail can be to some degrees affected by frequency response.
Minimising variations in response over the audible high frequency range (lets say for arguments sake, 3khz - 20khz) will result in that smooth sound you speak of.
Likewise, variations in response can also appear to hurt percieved detail as well. Premature roll-off in response above 15khz (a characteristic of most tweeters) will reduce your percieved detail in the upper frequencies. The result is that your sound will tend to appear somewhat dull, lacking sparkle and dynamics. Detail can also be effected by distortion levels and other factors as well.