Personally I don't find the Denon or Fostex series to scale with amplification all that much. Upgrading your source material and source (DAC/Transport) would yield better results. I've tried a wide variety of amps low cost to high cost and while they all improve on sound as you climb the ladder, the performance/cost ratio doesn't seem to scale all that much in comparison to other headphones that I find scale very well (i.e HD800, LCD-3, HE-6). I think the Fostex headphones in particular respond much better to synergy than to just flat out more expensive gear. A fine example of that would be the WA7 paired with the TH-600/900. Relatively low cost combo that sounds incredible together.
The grace m-901 gets a few mentions in the old skool threads but I'm more of an amp guy myself. Granted I've tried out more amps than dac's 20 vs.8 now but I've only come across 1 out of 8 dacs I have been impressed with and that's the keces da 151 mk2. The keces + rpx-35 combo wasn't a whole lot more than the wa-7. Perotta consulting has the hookup on the Rudi's.
While the wa-7 is an amp/dac combo, it's still a thousand bucks. If that is low cost to you I hate to see what you consider expensive.
Edited by jibzilla - 10/10/13 at 10:44pm