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.