Hi! Unfortunately, I found only pretty few Bluegrass/OldTime I really like (so far):
Allison Krauss&Union Station, Nickel Creek, Crooked Stairs, Tony Trishka…
I want MORE of that stuff (impressive fiddle/banjo/mandolin work, creative arrangements, if any singing at all, than NOT the traditional high-pitched stuff, rather sweeter - like in the bands aforementioned - and preferably female voices) - anything You might recommend? 
Bela Fleck-- any and all. On a different note, he has an album called Perpetual Motion which is Classical played on banjo. Very well too.
Crooked Still is the band that I think is doing the best job of renovating the Old Time stringband sound, with the whispery-voiced Aoife O’Donovan fronting a band that includes banjo, bass, cello, & fiddle. The cello is the magic ingredient.
In a similar line is Fiddlers 4, which is more of a super-group than a band - it’s three well-known fiddlers (Darol Anger, Michael Doucet, and Bruce Molsky) + Rushad Eggleston on cello. He’s a generation younger than the others, but he holds his own, and the cello gives the band something of a ‘chamber bluegrass’ sound, albeit with none of the stiffness that you can get when conservatory-trained musicians try to play music from a improv-based genre. Eggleton was also a member of Crooked Still for a while.
You may like The Cherryholmes.
Also, check out “Music For Two” with Bela Fleck and Edgar Meyer.
EDIT: Anything with Abigail Washburn is great. Look for albums by The Sparrow Quartet and Uncle Earl.
Each member of the band called “New Grass Revival”,
as well as the band itself, is worth looking up.
Cheasapike
Sam Bush solo albums
Punch Brothers
Andy Leftwich
Darrell Scott - not quite full total bluegrass, but albums full of great music.
Another good one is the Carolina Chocolate Drops.
you can go to woodsongs.com
and check out their archives
there descriptions there so you don’t have to dl all the concerts that there are there.
For female voices, and very fine playing, listen to the ‘Wailin’ Jennys’.
http://www.myspace.com/thewailinjennys
And then there’s this:
Don’t laugh until you listen to some of the samples. These guys are REALLY good. Funny concept but a terrific album
Hayseed Dixie rule!
The versions my band does of ‘Fat Bottomed Girls’ and ‘Feel Like Makin’ Love’ are adapted from Hayseed Dixie covers. Don Wayne Reno and Dale Reno from the band are the sons of the great bluegrass banjo player Don Reno.
Sam Bush is a good call.
Don’t discount some of the old stuff - there’s more to it than Old Bill’s “high lonesome” vocal style. Check out Monroe’s ‘Rawhide’. Killer track.
These guys are really, REALLY SICK! (Drugs or something? Is that, why they mention “grass” all the time??) Yet I hope, they’ll never get treated - the music’s just INCREDIBLE! 
As long as we’re talking about bluegrass music in general, my current absolute favorite is Split Lip Rayfield out of Kansas City.
Also, I really dig Nitty Gritty Dirt Band and Old Crow Medicine Show too! Good call on Union Station, although I personally could do without Alison Kraus. IMHO, I don’t know what all the fuss is about when it comes to her. She can sing, but her voice does not grab me like other female Celtic or country/bluegrass artists…
Matt
Well - she can sing AND fiddle AND look good all together! “New Favourite” is one of my old favourites… But then again - I prefer her rather WITH UnionStation than solo…
Try Hot Rize and anything by Tim O’Brien. He did one with his sister Molly called “Take Me Home” that is excellent. You might want to look for Peter Rowan and Emmy Lou Harris, too.
I almost mentioned Emmylou Harris and then forgot… Good call! 
Matt
Emmylou is just a wee bit too “country” for me…
Just discovered a young mandolin player, Sierra Hull. Great playin, beautifull singing - and she’s CUTE, too…
Well - with Your help, I managed to rise up my own little “Instrumental&female voice-nu-grass-archive” - Great stuff & cheers for Your help!
The Greencards.
…ordered…
After listening to all the other stuff You all recommended, I can only say: I am veeeery thankfull for having found this forum - obviously, people here really have a very good understanding of music and - logically following - a good taste! However - just a personal remark: I still seem to prefer Tony Trischka to Bela Fleck, even though I usually prefer the creative pupils to the more conservative teachers. Only here, they seem to be the other way round - but then again, that’s really a question of taste and if You don’t like jazz, some of Trischkas stuff will necessarily go on Your nerves… 
And then I discovered another band pushing the borders of bluegrass veery far - the punkgrass band “Uncle F**ker”. Well - strange stuff, some may even find it disgusting. However - without a doubt good musicians and creative stuff! If You’re courageous and like grass and punk, just try! Thanx however…