1. Added a PS Audio Power Plant Premier (ex demo model) a few months ago. I have to say that out of all the tweaks I've made, this one has made the biggest difference by far. According to the display on the PPP, our house suffers from pretty "dirty" power (over 6% THD). The PPP reduces the output distortion to around 0.5%. I'm not an engineer and don't pretend to understand power distortion, but I do know that the improvement to the sound was obvious and clear right from the minute I hooked it up. Money well spent!
2. More recently, I had both of my Linn amps upgraded to the Dynamik power supply. Not cheap ($750 per amp), but this also has made a clear improvement to the sound, so again, I'm happy.
No question that the Wireworld Starlight coax cable improves the Sennheisers dramatically. That said, I'm still not sure they are worth the money. I'll probably keep them due to convenience and laziness, but I can't say I'd strongly recommend them.
In the process of upgrading some cables. Just installed Audioquest Carbon HDMI cable from Mac Mini to Anthem. Waiting on a Wireworld Starlight HDMI cable which will go from DTV to Anthem. Also ordered MPC/Mini+ power cord for the Mac Mini.
Also just received a set of Sennheiser RS 220 wireless headphones mostly for watching TV when others are asleep (or being noisy), although I can also try them for music as I walk around the house, and control iTunes/Pure Music with my iPhone. Will report back after hooked up and broken in.
Mjcmt - seems like it works out great, but then I haven't really tried any other processors with these amps (and vice versa).
One nice thing about the Linn amps, aside from the sound, is that they are really lightweight, small and run cool. This is important for me since my system is in an enclosed built-in cabinet. My old Rotel behemoth would probably have shut down in this cabinet. I bought a thermostat-controlled fan system for the cabinet, but I don't need it with these amps. In fact, my Mac Mini runs much hotter than the Linns.
The biggest problem with my system now is that the room acoustics are wonky, and my subwoofer is too far to one side. Unfortunately, until we re-model our tiny house, I don't think I can do much about that.