Advice needed to get the most out of my Sennheiser HD600s

I have tried to work this out for myself by searching the forums and reading extensively, but there is so much conflicting opinion out there, even about the equipment I already own.

Please could some experienced members advise me - what ONE upgrade would they make to my current setup, in order to wring more performance out of my HD600s.

My affordably assembled (by Head-Fi standards) rig is:

HD600 with stock HD650 cable

Audio GD Fun amp / dac with Earth OPA module fitted

Source: lossless audio files stored on PC and connected to Fun via usb

My musical tastes are wide, from classical, through electronic, to metal.

I very much enjoy my music through this setup, and can listen for hours at a time. I like the sound signature I'm getting. My ears dislike 'shrill' highs, and I dislike flabby bass.

Budget wise, I'm willing to spend money (but short of summit-fi), in order to improve the signature I currently enjoy, but can't figure out where best to target it.

Any advise you can offer would be much appreciated, and my poor head will thank you!

If it helps, the upgrade paths I've been considering are:

1. Using the Fun's dac section to feed a better amp

2. Getting a better dac

3. Improving my source, but wasn't sure where to even start with this

4. After market headphone cable - although I'm dubious of the benefits, and even if I weren't, would there be any appreciable difference within a modest setup anyway?
 

Thank you both. I've never tried a tube amp with my HD600s, only solid state.

I appreciate that more power will only improve the sound signature I love. What impact are tubes likely to have on it?
 

Thanks Nirvana, I know I'm new here but I've had this setup for way over 2 years, and owned several different headphones (HD650, Denon D2000, Grado SR225i, Beyer DT880 for example) and amp/dacs previously. It took time to find exactly what I like and now I'm just trying to get the most out of it.