Finally got around to trying Markun's HQ patch which has been around for a few years now and from looking at the code - it appears to increase the Headphone Bias current by 50% which is supposedly helpful to drive headphones of a higher impedance.
I must say that for quite a while I've been underwhelmed by what I perceived as being a less than full and a comparatively colder, less powerful and somewhat spiky bass sound of Rockbox to that of the Official Firmware through the headphones which I use. I mainly use headphones of around 60 -70ohm. I've never had a lack of 'volume' in either firmware but felt that the Offical firmware tended to drive my phones better and others that share my ZIP and headphones are of the same opinion. This patch works great. I'm now for the first time starting to enjoy Rockbox.