I've played with old Fender tube amps from the sixties at the beginning of my "career". They belonged the father of our drummer and I had not the least idea what they're worth way back in 1987. My Ultra had been really nice, but with the II I experience the same "ringing noise" like with the real amps. To be honest, I forgot about that special sound until I heard it again in my II...
When I tweak the basic sounds, I usually do that with headphones (since some weeks a Bang & Olufsen H6, they're the best closed headphones I've heard until now and I know and own Ultrasones, Beyers, Audio Technicas, Sonys, too. Give the H6 a try!!!). Last night I played around with SLO100 with SW 11beta and suddenly stopped for a second without "actively" thinking why in that moment: This was because I was not sure whether it was really me and my old guitar! I've never experienced something like that before!