I was there too and tried the TH500 with my own gear (iPhone and CEntrance M8), so there was no lack of bass. In the tracks that is. The TH500 does not go very deep and is missing the punch I'd like to hear. I can only second what MrSpeakers had to say about them: he told me that the drivers are really not suited well for an open headphone. It was very interesting to compare them to the Alpha Dogs, which do very well in that regard.
I would characterize the TH900 more as U-shaped, not classic V
Yeah, they are rather flat, except for the rolled-off deep bass. I wouldn't call them mid-forward (that is reserved for the AT-like LCD-XCs). Beats Antique, for example, comes across like it is being played through a pair of studio monitors and lacks impact, but is very smooth and non-fatiguing.
I'm using them mainly from a Geek Out 1000 with an Aurorasound USB PSU, which seems to be a decent match. Unfortunately I can't modify them to run balanced, as they'd probably work well with the Centrance M8.
Sonic description matches my impressions quite well. I'm somewhat reminded of the drivers age hearing these... the signature is in line with that, while MrSpeakers does
quite a good job pulling them up to the present.
They did work nicely with the M8, though I used the unbalanced output. Not very sensitive, but middle gain setting worked for me.