Nice, I copied your example and got a shielded audio cable, and I agree, threre's a noticeable improvement. Although I think everyone has the hissing issue over a certain volume?
I've also added a Powerblock (http://blog.petrockblock.com/2015/07/04/powerblock-another-power-switch-for-the-raspberry-pi/) which is the version of your homemade power switch for those of us who are electronically inept!
Works well, but you have to add your own switch and LED. I combined the two and used an illuminated latch switch (5 for £2.35 from a Chinese Ebay seller)