Anyone tried the Benchmark

This is a solid suggestion. This will make a great computer based desktop system.

Thanks for the reply. If I'm used to tubes, what immediate differences will I likely notice going to solid-state powered monitors?

This is typical studio equipment. So you will hear the music as intended by the mastering engineer. Some mastering engineers (like Doug Sax) use tubes so the sound of vocals is already sweetened and the drums may have more weight/punch from the added compression.

However, if you like to sweeten things even more with your own tubes then you may find the sound to be thinner, more neutral and with less warmth than what you are used to.

I have the HD-650's and had the Benchmark DAC. I also used the DAC-1 with my Krell KAV-400xi/Dynaudio Contour S3.4 speaker setup. FYI, Head-Fi.org has a ton of info about the headphone pairing with this DAC and others.

What I can tell you about my experience is that I didn't care for the sound. To me, it was harsh, analytical, forward and cold. My opinion applies both to the speaker setup and the DAC-1 directly driving the HD-650s. Sources I used were Sonos ZP-90, Apple Airport Express and my MacBook Pro digital out - admittedly not the greatest "transports" but I always used lossless files. I went to an Electrocompaniet EMC-1UP CD Player and wow, what a difference - warmer, definitely, but a much more realistic and enjoyable experience. I sold that and went to the ECD-1 DAC, which is basically the digital section of the EMC-1UP, and it still sounds great, but lacks the smallest bit of extra magic as compared the single-box CD player - probably attributable to jitter from the Sonos, cabling, god-knows-what - but it's still much better to my ears and in my system than the DAC1 ever was.