Yea, HD650's drivers are definitely capable of producing a lot of bass, definitely more than HD25-1, it just that they're very neutral cans, they won't boost bass in music. They will reproduce as much bass as the song has, but they're definitely able to produce crazy bass with songs that have artificially boosted bass. And difference is, HD25's have a very in your head bass, while HD650's have a big bass, more similar to the type of bass that big floorstanding speakers produce.
By the way, I see you have Woo Audio 6 now? Did you notice a lot of difference between it and pro-ject amp? I'm planning to buy a better amp in future for my HD650's just to see what they can do when they're working at full potential, if its worth it.
yeah hd 650 has more of that big clean bass you get from an expensive reason vs. that punchy in your head bass. the hd 650s are just more refined and go deeper too i believe.
as far as the amp, at first, i didn't notice as much of a difference as i'd expect. the pro-ject amp is very very good for the money and i personally think its underrated. upon burn in however, there was a big difference, there was more detail, imaging, and more dynamics to the hd 650s. while the proj-ect amp offers 80% of the woo wa6 in terms of performance and overall quality, the wa6 is still on another level. it makes the hd 650s slightly warmer, but also alot smoother and more enjoyable to listen to while maintaining really good dynamics.
so if 20-25% improvement sounds reasonable, go for it, you'll love it. but if you're happy with the pro-ject and feel that spending 1100$ on the wa6se is a bit too much, just stick with it for now and get a good dac or something instead. when i got my dac, my project amp suddenly sounded way better because the soundcard i used while also being decent, does not compare to a 500+$ DAC