Thiel's benefit greatly from valve gear being in the chain. The AR-10 pre (its very good - but pricey) sounds really good with them - a bit dry otherwise.
That said the pre's that will do justice to the Thiels, like the AR-10, are very pricey. My advice is direct connect it and save up for an AR-10 pre or something similar.
I also have to say I don't think the Direct Stream is the best match for the Theil's - valve based DAC's would be my choice. Here is a comparison of am EMM DAC and a very good valve based DAC, a Killer, on Theil's:
I was able to use Bill's Offramp to feed I2S to the Killer and AES to the EMM Labs XDS1 simultaneously and was able to A:B easily between the two using the ARC preamp.
The EMM does a lot of things right for me. But the Killer sounds more natural in the end. The XDS1 seems to exaggerate differences - higher highs and lower lows but the end result (which on its own doesn't sound bad to me) is less listenable to me than the Killer in the AB. I loved how the Killer reproduced guitars in Fourplay's 101 Eastbound (HD Tracks) which made it "easy" to follow. Where the XDS1 reproduced the music, the Killer sounded like it was an organised quartet led by the lead guitar, with the rest of band accompanying him. I really enjoyed that quality.
When we listened to the Phasure a while back, there was a similar disconnect - the Phasure reproduced voices like Pat Barber with a raspy or gravelly quality - was it more detail? I thought I heard that with Joni Mitchell's Both Sides Now (DVDA rip) on the XDS1 today. The Killer was smoother but more natural. I've only heard Pat Barber once, in a club in Chicago many years ago and from memory, it seems to be somewhere in between. It wasn't quite buttery smooth, but it wasn't quite as gravelly as what I heard on the Phasure either. If I had to choose, I'd err on the side of the Killer.
When I first heard about the Killer, about how well it sounded with vocals, I can't say I was particularly keen to listen to it. I had written it off along with many warm syrupy slow Chinese tube amps I had heard int he past. But it wasn't the case. I even threw in a bit of Depeche Mode for good measure, along with FourPlay. Over the week, I am sure I can put it through the paces a bit more with some of my more familiar material.
If there is one downside, and actually it was more of a technicality, it was that we couldn't play hires material with it. But when we were playing HD Tracks material, even with the OffRamp downsampling to 44.1, the characteristics I liked about the Killer were still apparent, even when compared to the material in native hires on the XDS1. Maybe there is something to be said about hold outs like georgehifi ;) who insist 16/44.1 is good enough.
BTW its not a spiel for you to get a Killer - its doubtfull you couls get a hold of one to check anyway - they are hand made in miniscule quantities - its just I think valve DAC's go well with Theils.