After getting the Hifiman HE-500 and getting accustomed to it's sound signature, I would definitely agree with you that the HD650 is not laid back sounding. Again, my reference point is the HE-400i, which has an upfront presentation.
I also agree that the HE-500 and HD650 actually have a very similar sound signature, albeit the HE-500 has I think a better treble presentation, making them more engaging listen than the HD650. However, I still use the HD650 during times that I just want to listen to music as a background and not purposely listen to music. The HD650 is still an excellent sounding headphone, hence why I still use them on a daily basis. They're just not as great for EDM or songs that has a lot of sub bass as the HE-500s are for those songs.
I think a planar headphone would be a great complement for the HD6XX (Hence why I have a Hifiman HE-500 on hand). I would definitely look at headphones that can provide good sub bass, so Audeze, Hifimans, or even Fostex types of headphones should fit the bill. Beyerdynamic DT1990 is non-planar, but based from what I've read (never had them), they might fill the good soundstage type of presentation you're looking for.
It would be good to look up on all the common offerings for those brands I've mentioned (i.e. Audeze LCD-1, LCD-2 variants, Hifiman Sundara, 560, Ananda, Fostex TH610 to name a few) and read about what other people's experiences are with those so you get a good idea on how they would sound and you can narrow down your choices based on your budget and type of sound they present.
Thanks. I also don't want to break the bank really, a certain planar like a modded fostex t50rp should be fine. I like the cheap sennheiser hd-25, the dj headphones, very nie bass signature, only complaint about those is the on ear design instead of over ear. Anyhow, are there really decent planar's for under 500€?