600 used speakers?

You do not need a sub with the CDM-1 (or any speakers), just decent stands and the right cables. Although a sub is usually better (with all speakers). The CDM-1NT are fine, just not as musical as the original. The CDM-1SE ranks third. They are good for HT, music is OK.

Used Vandersteen 1's would be a great choice. I have been a long time owner of the 2C (both 2c and 2ce) but would not recommend them unless you are willing to spend some money down the road to drive them appropriately. The Model 1's are a much easier speaker to drive. As an alternative, I would suggest B&W DM602 S2, which you could get new for $600. You would have to add stands for about $60 or so. The B&W's will not go as deep as the Vandy's but they have a nice midrange and they look a little better.

Sean's first paragraph is right, and you've got higher fidelity equipment than alot stuff out there. There's definately better, but I feel you could have paid alot more money for alot worse sound. I'm a bit jaded, but I don't even look at 90% of what's out there anymore, at leat not frequently. You can get very good fidelity with your gear. In the end it all matters, DAC, pre, amp-everything, but in the end the speakers still get the vote as the most important player. You could buy used and probably find a good deal. If I was in your shoes thoughI'd take the DIY route. www.murphyblaster.com has a very nice two way (the MB1) that can be built for $350 in parts. Its using a www.gr-research.com mid/bass driver with the Hiquphon OW1 tweeter (don't know thier url, got to zalytron.com). With the money left over you can even get a nice diy sub and have very good sound. Its got a ruler flat frequency response and that tweeter will be fleshing out more detail than anything used in that price range. The only real downside to diy is you may not have resale value, but you can always move them to the bedroom or for surrounds depending on the future. The Hiquphon tweeter it uses is very nice, and a bargain for its price. And you'll walk away with the most rigid well-braced, non resonant cabinets around if you want'em to be.
As far as used, I've never like NHT, heard the biggest, owned the Superzero. Their a step up from some stuff but not that great. If I recall they pay less than $20, more like $14, for the tweeters in their $4,300 flagship. I've always though B&W sucked. Of course I didn't start hearing them much 'til after I owned my L/1's. The 805 is still overpriced on the used market and while the 801 may have been good in its day, it is getting old. Haven't heard Vandersteen, and they're probably better than alot of stuff, but it does come down to parts cost that do limit just how good things are. Half of a speakers MSRP is dealer profit (+/-10%), and another chunk is the manufacturer making their profit and paying the cabinet maker etc. Over on one diy forum one guy took apart a Monitor Audio center that cost $950 new, and its got 20g speaker wire, not MDF but not Particle board cabinets either, little bracing, two mid/bass driver that were using cheap plastic frames and less the perfect coil windings. PSB made some nice stuff. I don't know about the new image series. And vandersteen is worth a listen. Generally though, a well-designed bookshelf will give more bang for the buck in your price range. But Dennis Murphy's site is nice with alot of good info, as is zalytron.com. They've got alot of drivers. The Birotechnology.com site has lots of nice articles too. Check out both options. But I would avoid anything with a metal coned driver. I know Theil is using them on everything now and I'm a bit disappointed. I owned their little CS.5, but there are far better speakers than it for the same money, which is about the only one of thier models in your price range, maybe the 1.5. (but I believe those are vifa tweeters in both models, which aren't bad, but most of thier models are not Scan-speak, Hiquphon, Accuton, caliber) My/the Biro L/1's would kill it, and most other things at several multiples the price. VMPS has some nice subs for cheap and you can dig around on DIY audio for some nice subwoofer projects, they're pretty easy.

OK Mctuff, you owe me for this one; I got ya all fixed up. For your sub try this Transmission line diy'er:

FREQUENCY RESPONSE (4 pi measurement): 10 Hz to 240 Hz +/- .8 dB
EFFICIENCY: l watt R.M.S. = 95 dB SPL
64 watts R.M.S. = 113 dB SPL
MAXIMUM SOUND PRESSURE LEVEL: 113 dB SPL @ 1 meter on axis

$300 +wood and labor at http://www.hogheaven.com/diyaudio/subwoofers/PATL/patl.html

I knew you needed a new coffee table, build some nice minimonitors and your set. Seriously though, I think 10hz is a stretch, but its still probably respectable peformance. Those TL's, if I recall, extend the low frequency resonance of the driver another half octave...down. Just shovel the extra $50 bucks from your cable fund, you'll never miss it.
http://www.diyaudio.com/forums/showthread.php?threadid=1701

2nd the Kestal floorstanders and look at Epos M12 standmounts.