So the input voltage of the GEQ is 12 volts?
To reduce the noise on that HDD power supply, put a 100uF+ cap across 12v to ground. make shure the cap is rated for 15+ volts, and if it is an electrolytic cap you match the palority.
What about your computer powersupply? ATX spec requires less then 120mVpp noise, and any good manufacturer runs at about 80mVpp
For low a noise PS you need a Linear power supply, which has a ripple of 4mVpp. A switching PS for your needs is about 120mVpp. Follow the link for a small linear PS, please check the dimensions in the data sheet, under case type B.
http://www.mouser.com/ProductDetail/...V7BK3RezC40%3d
You can also use a DC to DC converter, which has a 75mVpp. the link is a 12volt in, 12 volt out.
http://www.mouser.com/ProductDetail/...F6h9ilfp77c%3d