Showed up at my doorstep today, and in perfect working order. Powered it up and dead silence in a good way, no hummm, or noise what so ever when turning the pot to max. My Aficionado had a tiny bit of noise, even with it's own power supply stowed as far a way as I could stretch the cable... What kind of sorcery is this? After 20 minutes of warm up time, I fired up the first test track on my bone stock HD600, Mötley Crüe's "Kick Start my Heart". Lord almighty, I was not expecting this thing to belt out Vince Neil's voice with such clarity at this price point. The SW51+ had my complete and undivided attention firmly in it's little grasp, and I've been listening almost non stop for the last 3hrs. Like most of the previous impressions have already said, this sucker sounds very SS, and not bloomy as in a stereotypical tube amp in this price range. I'm extremely impressed with the texture of the treble using the stock tubes, it doesn't sound rolled off, but exhibits no glare with the HD600, tons of nuance and microdetail. The mid range is also silly good with the HD600, and the low end isn't too shabby at all. The HD600 is rolled in the bass department anyway, so I tried my uni-cough, cough-corn LCD2, and the bass was solid, and linear, but lacked the mid and treble magic of the HD600. I also like the party trick this amp can do... by plugging my HD600 into the H output it changes into a HD650 sound wise, moar mid bass, lol. I briefly tried some NOS Phillips military toobz from the 80's, that I got for a song ($25 for a sleeve of 5). I've always found NOS military tubes from the 70's and 80's solid, reasonably priced, measure extremely well, with minimal noise, and generally lack microphonics (Philips military are usually green print when made in the 70's and blue in the 80's). They sounded a little fuller in the mid range, but will run them in at night over the next week before giving them another listen. The stock tubes are excellent in my opinion. To me this amp embodies the spirit of SBAF and the hobby community, high value, and BIG performance!!! Thanks to @Zampotech, @Hands, and @atomicbob, as well as the countless others for bringing this amp to fruition, developing, measuring, and just getting the word out.