The crack with speedball will be a big change and a major improvement in listening enjoyment with hd650s if your ears are anything like mine. I actually like my agdr boosted o2 quite well with alpha dogs and other fostex t50rp variants. It is also pretty good with dt880s since they have kind of a diffuse presentation and the o2 helps to focus them and give them a nice impactful, albeit slightly lean sound. But hd650s are already very coherent sounding headphones as far as instrument separation, and the o2 just makes them sound compressed and rather unremarkable in comparison. Timbre also takes a big hit.
Plug into a crack and you will immediately understand the hype. It is just an excellent pairing that brings out the 650s soundstage, yields rich full lifelike timbre, and draws you right into the music. It is not a slow tubey sound like the woo otls. It is a punchy, dynamic involving match which will also hit harder and provide more substance in the bass than your o2, although at the cost of some bass tightness (which you will not miss in the least on balance). Building the kit and getting to participate in the extended bottlehead community is also extremely rewarding and is a big part of what has kept me interested and active on headfi for all these years. Plus, as Doc always says, it really does sound better when you build it yourself.
Definitely satisfied and enjoying the music.
