Advice: used speakers $600


Choices: PSB stratus silver,Kef 104/2, B&w Dm7 mk2...any clear cut winner? I prefer a smooth, balanced sound, open midrange, controlled bass over extension.Also, which one of these requires the least maintenance in terms of upkeep.
phasecorrect
I've always like the KEF sound, I'd vote for the 104/2s...

-rw-
Why not demo the Silverline Minuet Supreme +. Around $700. One of the best values in audio. Great midrange,pace and tone.
Why get older speakers that may need to be reconed, capped, etc.
http://app.audiogon.com/listings/full-range-green-mountain-audio-europa-2015-01-26-speakers-95409-santa-rosa-ca
I'd probably go for the Stratus Silvers or Stratus Gold if you could find those
in your price range.

However, if you can find a pair of Mirage M5si's at $400-700/pair, get 'em!

I have a pair of brand new Magneplanar 1.7s anchoring my 2-channel
analog-sourced living room system, but in the family room, I still have a pair
of 1996 Mirage M5si's anchoring my HT system. And you know what? Those
Mirages would still aquit themselves in a 2-channel music-only system. I
don't sense any loss in musical truth when I go downstairs to hear the
Mirages.

These Mirages have a smooth treble, lifelike transparent midrange, and fast,
tight bass that extends down to the mid-20s. True, the Magneplanars are a
bit faster and more transparent, but really, the Mirages come surprisingly
close while having a broader dynamic range, another octave of bass
extension, and energize a wide range of room sizes with authority.

The Mirages will reward you with better cabling, amplification, placement,
spiking, etc. But they are also very durable. Mine are almost 19 years old
and have had no service done on them. The mid/woofers have cast baskets
and butyl rubber surrounds. The tweeters are cloth-damped titanium domes.

In 19 years and daily use I have not found anything to not like about them. If
you can find a pair, get 'em!
Stereophile described the PSB silver as bland...but gave class A rating to Gold... Which is essentially the same speaker but bigger... Funny
There's a pair of Silverline Preludes for sale here a little over your price range that have the sound characteristics you are seeking and are better looking than the Kefs and definitely the B&Ws -- Ick. Best of luck.
Monitor Audio RS-6 is a great speaker and grossly underrated, except for Bob Reina of Stereophile who used them to compare other speakers. It is solidly built, and has the cleanest bass of any speaker under $3000 I have ever heard.

You don't see them on Audiogon that often but depending on condition you should be able to pick up a pair for $650-700.They originally retailed for $1100

As Mr Soix noted the Silverline Prelude II (not the I) is also a very good speaker, but not quite the equal of the MA RS-6. in my opinion Good Luck
Heres a really good one
http://app.audiogon.com/listings/monitors-audio-concepts-aci-sapphire-25th-anniversary-edition-2015-02-14-speakers-11209-brooklyn-ny
Second the Minuet Supreme Pluses for a small room, Preludes for midsize room. You will get midrange, pace, and tone in spades for little money.