Definitely true that there's a lot of variables in the KGSShv builds, and therefore sound. It evolved/changed over a few years too:
- Power Supply: Full or minified
- Transistors: IXYS, Sanyo
- Voltage Rails: 400V/450V/500V
- Bias current (5mA to 14mA+)
- On-board vs. off-board heatsink version
- DC Servo vs. no servo
- Passive Parts (e.g. attenuator: Alpha pot vs. DACT/Goldpoint vs. RK50)
So, lots of variables over lots of builders. Hearing various head-to-head match-ups between 5 different KGSShv builds, over time, is pretty striking. They do sound different.
Amp selection here is a much bigger determinant of overall tonal balance than the DAC. Headphone selection being first of course. So try to hear a 007 and/or 009 before you buy. Or take a shot at one with the idea that you can resell if it doesn't work out (let's be honest -- if you made it here, you're dedicated enough to do this much at least). To be honest, I love the 009 so much that I still like it a lot even out of a too-bright amp/DAC -- it's just that the 009 is much better with an ideal match. Haven't heard a modern 007, but from what I've read I'd almost certainly prefer it over my Mk I. I prefer my L700 over the 007 Mk I, even though the latter is better at detail and soundstage (I don't give my 007 much love anymore) -- the L700 is pretty much dead neutral to my ears; great balance and musical.
I've heard three 009's (spanning at least a few years of makes) and two 007 Mk I's at the same time (between Purk and myself); unit/vintage variability is very small -- the oldest 009 happens to be the warmest, actually. The channel balance is always perfect. My impression is that Stax is extremely consistent -- especially versus other non-Sennheiser manufacturers.
Here's a rundown of my thoughts on various amps:
- KGSShv Carbon (17mA bias, Goldpoint): Very close to neutral; slightly laid-back in the upper mids and treble, which pairs well with the 009 and Yggdrasil. Absolutely huge soundstage; amazing imaging.
- KGSShv off-board 450V Sanyo full-size (11mA bias, Alpha pot): Surprisingly warm and lush. Not as detailed as the IXYS KGSShv or Carbon, but maybe some of that is the Alpha pot vs. Goldpoints. Still very detailed and dynamic, with great bass impact and soundstage (kills the KGST in these areas). This will be the perfect 009 pairing for those who find the latter slightly too bright/strident; it reins in the top-end more than any of these other amps (stereotype of tube amps being the warmest does not hold here). Not an ideal match for the 007 Mk I; things get a bit wooly there.