What the benifit of using a separate dac?

Good points, Blindjim. There are a handful of DAC's that also convert USB directly to I2S (native language to the DAC) which circumvents the SPDIF interface (an additional conversion) in between. I think Empirical does a conversion to the Benchmark to accomplish this, as does the new Redwine Isabella in stock form, as well as others. Just instinctively keeping the signal down to as few conversions as possible would seem like a good thing. I'm not sure if this is similar to "asynchronous" USB conversion that is utilized by Wavelength in their recent versions of DACs, but I think these technologies are doing alot to address the de-jittering, as Jim put it, and computer>DAC connection and the clocking issues therein. Each manufacturer seems to be pursuing their own approach, and ultimately they all present a slightly different flavor. I was astounded in comparing DAC's recently, just how differently they can present music. The fact that so many love the Benchmark and I found it un-listenable (the highs were just too strident for me) just supports what I've always said - it is just as subjective as anything else in this hobby and you really just have to use your own ears and your own system/room/music to really make a judgment for yourself.

Jax2 - It depends when you listened to Benchmark. Early ones had Signetics/Philips NE5532 Op-amps that sounded thin. Around 2001/2002 Philips had factory fire (burned down) and stopped making them. Texas Instruments bought the license and designed new larger die that sounds better. Benchmark also needs a little time to get civilized but even after hundred hours doesn't sound warm (if that's what you're after).

Kijanki - I listened at length to two versions. One was an early version, but post-2002, which was thoroughly burned in. I listened in three very different systems to that one. The second was more recent and was a current, USB version which I ordered direct from Benchmark. Again, I listened in two different systems to that one. Since this was from the factory I had to burn it in myself. By the time I returned it I'd estimate I'd put around 200 hours or more on it by leaving it on whenever I could, so I don't know if it could have sounded better given more time. I don't know whether it came from the factory with any hours on it since they do have the return policy. The latter version was definitely an improvement over the former, but still suffered from stridency in the high end which ultimately made it difficult to listen to for long periods. I found it on the colder side, a sound I'd associate in some ways with SS amplification. OTOH, I heard it in several systems at RMAF and I very much like what I heard there, but I did not listen to any one at great length to determine whether any similar traits to what I heard at home existed in the long run. I have no doubts it can sound good to some folks - clearly it is praised by many - it just did not sound good to me, rather, in my systems in sounded clinical and a bit harsh. In comparison I much preferred an MHDT Paradisea which presented the same music with a much more natural presence. The early version I had I compared to a Wavelength Brick and a very old Muse Model 2+ both of which I preferred, the Muse being more similar to the Benchmark, but not nearly as harsh, while the Wavelength was a sharper contrast being a kinder gentler presentation. I believe all the DAC's I preferred were NOS DAC's if I'm not mistaken. I also tend to like DAC's and players with a tube output stage in general, though not as a rule. As with everything here, YMMV - to each their own.

Thanks so far for comments, its hard when it comes to digital, I live a long way from major cities, so trying in home is impossible, I buy all my gear from all over the world as lot of the brands are not available and plus work commitments have there restraints on my time, so far I bought everything I have using reviews and chat on Audiogon and have been very happy so far.One last question if a Wadia player is so good why do they have to be moded?

One last question if a Wadia player is so good why do they have to be moded?
K_rose

Its not that Wadia product HAS to be a mod candidate,but what the audiophile desires, in his or her particular set-up.
Wadia, like any company, has price constraints when a design goes to market. If the moding company(GNSC comes to mind) can make a Wadia sound all the better, then IMHO it is money well spent. Think Synergy...