I seem to remember hearing that the A85 has a good built-in headphone amp. Anyway, if you get the Slee Intro or Solo, you will need to feed it from the RCA outputs of your cd player or phono stage, or the amp tape loop output (which is further down the path away from either source but is certainly the most practical).
The Benchmark DAC-1 is great with Sennheiser HD560s by the way, and it's at a similar price new as the Solo, plus its DAC will transform your cd player to the high end. Nothing has ever made more of an improvement to my system that the Benchmark, it is an exceptional piece of kit.
Another thing about the Senns is that you can replace the cable - it pulls out if you yank - with a Cardas cable and this is supposed to bring a significant improvement. I never bothered as I was using them mainly at work with a portable usb dac.
Yes, the GS-1000s are amazing. The main thing is that it really doesn't feel like you're wearing headphones. It feels like the sounds are coming from outside. Also it is by far the most comfortable and unfatiguing headphone I've tried, which is much more important than you'd think. But I was a little taken aback by their balance which is slightly cold, and I like my sound a bit warm and lush (I LOVE the Sugden sound). My only good source so far is the Benchmark DAC-1 and they don't mate that well, as they are both ruthlessly analytical and a bit bright, meaning that any sibilance or poor recording is painfully obvious. However, I have a tube headphone amp, the Earmax Pro, arriving shortly, which is supposed to have a very smooth, sweet, mellow sound, and therefore should be a good match.