Alright,
I finished up my sierra center Nrt upgrade and spent some time listening to it last night. First off, I had the old cabinet, so installation was a bear, but I managed

Setup...
I brought down the sierra Nrt into my theater room(lots of sound deadening paneling and a yamaha receiver) and setup one center on speaker A and the other on speaker B of my receivers input, both left channel. I then was able to use the remote to toggle between the two center channels to compare the differences.
Like pj, my first track on my own custom demo disc is Nils "Keith dont go". First thing I noticed was the extra top end sizzle, the guitar plucks and etc had more sizzle and energy. Then switching back to the original sierra, it sounded dull or veiled in comparison. I felt the guitar notes on the Nrt sounded much more realisitic regarding timbre accuracy compared to listening to a live guitar. I also noticed the male vocals sounded much more forward (more weight to them), where as the original sounded a bit boxy and hollow. As I listened to some other material (Jack Johnson, Dire Straits, Jesse Cook, and Fleetwood mac) I realized the imaging / soundstage was quite different among the two. I came to the conclusion that the Nrt produced material with an "airy" quality to all the notes, it made it seem like the soundstage grew and the imaging was more pronounced. Cymbal notes hung in the room longer while the original sierra seemed to play the note and it was gone. The sound of the Nrt was a large improvement in my mind. The extended high frequencies were very noticeable without any trace of sibilance. I can attest to Dave's measurements and how far the Nrt tweet extends on the top end!

Overall, I was most impressed I was surprised at the male vocal improvements as the midrange became more forward and fuller in my mind. The cymbals, hi hats, and guitar plucks all sounded so much more accurate, were more pronounced, and seemed to float in the air with better separation with the other notes being produced.

Congrats to Dave F and crew, the Nrt upgrade was well worth the price of admission Normally I am not one to notice subtle differences in the audio world(nor am I an expert) but the differences were very noticeable when comparing them side by side.

Equipment -
Yamaha Rx-v1600
Toshiba 4800 DVD player - source
Treated theater room, great for mids and highs, although I have no bass traps, so the bass is overly emphasized. Fine for movies

Tracks-
Nils Lofgren - Keith dont go, man in the moon
Daryl Stuermer (Genesis band player) - man on the corner, in too deep
Fleetwood mac - Go insane live, family man, and seven wonders
NIN - into the void
Santana - corazon espinado
Jack Johnson - rodeo clowns, upside down, breakdown
DIre Straits - ride across the river
Rob Thomas - all that I am
Jesse Cook - switchback, virtue, cafe mocha

Next on my list.....

Arrival of my Ascend Towers, name TBD


Thanks!
Brandon