My G109 puts out a ton of power & voltage and generally has really good bass. The HD600's bass did improve slightly when I moved to the G109 but they are still bass-light in comparison to most of my other cans though usually only slightly. I wouldn't even characterize the G109+HD600 as bass-light really, just not emphasized at all in the bass, relative to the other frequencies (neutral). If I use the HD600's exclusively I quickly get used to their signature and the bass quantity is just about perfect. It isn't until I compare them directly with slightly bassier 'phones like the K712 that I notice they are a bit bass shy. The bass is there when called upon and all of the details are there but it stays out of the way of the other frequencies. So if you like bass emphasis they probably wouldn't satisfy you. Personally I really like their signature. It could use just a hair more very low bass extension but overall I think the smooth and neutral signature of the HD600 does a lot for it, in a lot of ways, and it keeps them interesting and satisfying despite the "boring" neutral presentation you hear initially or in comparison to some other cans.
I've heard the Crack increases the bass response and I can't wait to try it out. I'll be building one soon. As far as SS amps I'd say don't expect the actual quantity of bass to increase or the HD600's sound signature to change markedly. For that look into EQ'ing. I've done a lot of EQ'ing with the HD600 to great effect. With proper EQ you can get as much beautiful, clean bass out of them as you'll ever need.
It's the natural sound that I like. I get what was intended by the recording. IMO the bass isn't light, it isn't exaggerated.
High output voltage + high output power + high damping factor/low output impedance + low noise make for a great amp. Some more than others, but at one point the differences are purely academic.
Try a FiiO E12. When you want it, the bass boost switch works well with the HD600, keeping to the sub bass, not affecting the mids, etc. The output voltage hits 15.5 Vp-p which is enough to hurt and the output impedance is < .5 Ohm which for 300 Ohm cans, not an issue for either output current or damping. This thing is a steal.