This is most odd! I had the 007s and now have the 009s and the 009s are miles ahead on everything.
Before I got a KGSShv to power the 009s (which makes these phones sing for sure) I used the 009s on a Stax 717, which is as good
or better than the current 727, and no doubt better than the 323 partly as the trans are bigger and parts quality higher.
The 009s on the 717 was ok, but really revealed the limitations in the source and amp. It was not really that great IMO. When you get a world class revealing headphone like the 009s you are really digging deep into the source and revealing all it's warts and all. I would hazard a guess the extra bass you here on the 507s is bass bloat in the phone. The 009 has KILLER bass when fed a good signal and decent amp.
What would I do? I would decline the KimiK amp, keep the 009s and get a KGSShv from Birgir or other source. To me, none of the current Stax amps have the quality or drive to do the 009s justice, sorry. I know this might not be what you wanted to read, but to me it is a fact. I got a DIY KGSShv for £1,700 and it kills any Stax amp. The Stax tube amps are an odd ball design, with weird and flawed current delivery, the 727 has that aweful feedback thing going on, the 323 is built to a budget but not up to the job of the 009s.
If you are really serious about a Stax system, sell the 007s and the 323, decline the Kimik and spend the money realised on a KGSShv DIY, or a WOO WES, Eddie Current or BHSE if the money stretches. There is no Stax amp that you will be happy with for long IMO. BTW I listen to Rock, Dance, Classical, Vocal, it all sounds awesome on the KGSShv with the 009s. I kept my Stax 717 amp for a portable system. It sounds brittle, cold, not real, edgy compared to the KGSShv with the 009s. It has an unatural edge to the treble that gives the perceived idea it has more detail, where the opposite is true. The rest of the Stax amps are the same or behind the 717 IMO. Drop the idea of a Stax amp if you want a top system and keep the 009s! Actually I heard the 507s on the KGSShv and they are very good for the money, but again, way behind the 009s.
Edited by astrostar59 - 9/2/14 at 6:37am