headphone amp dilemma (w/ senn HD600s)

It depends on the amp. Some will bring out the bass while others won't do it as well. The HD-600 (great headphone, by the way) is sensitive about what amp it's plugged into.

What kind of sound are you looking for? How much do you want to spend?

I have the HD650s and a Burson HA-160, this combo I think is fantastic if you like a smooth, powerful and meaty sound. To me there is a night and day difference between powering with a real amp and without, sound has more weight, bass is tighter and the stereo image becomes much more defined.

 

The HD600/650 I think are fantastic headphones which scale very well, as you update your source and amp they will continue to do the system justice.

 

Some headphones can suffer from weak bass when they do not have enough power.

 

The M^3 and PPAv2 are both headphone amps with a bass boost knob. Either one should work very well with the HD600s. They are not commercially available, so you will either need to D-I-Y it, or commission a professional builder to do it for you.

 

The bass boost is adjustable. And also optional, some choose not to add the bass control at all. My M^3 has one and I think it's great.

 

Because of the custom built nature of these amps, the price depends on just how decked out you want your amp to be. For instance, a genuine OPA627 opamp chip costs $25 a piece, and there are 3 of them in each amp. Or you could go for the AD8610 chip instead, which is also excellent but costs $10 a piece.  Then there is the power supply. You could use a wallwart to save money, or spend on the sigma11 to squeeze out the last few percent of hi-fi goodness from the amp.

 

M³: http://www.amb.org/audio/mmm/

PPAv2: http://tangentsoft.net/audio/ppa/

 

You can try contacting MisterX or fishski13 (Solderworks Audio) for a quote. Or anyone in the DIY forum here who has a "Member of the Trade" badge below their username.

 

I have seen several m3 in the for sale forum recently. Not sure if they are still available but that could be a good option.

 

I think the bottlehead crack is a great entry level option for sennheisers. It works great with my 600s, though I have not heard it with 650s. It is well within your budget, even if you have bottlehead build it for you. It does not have a bass boost however.

Instead of analog bass boost in an amplifier, you can also try using a digital equalizer. I'm sure your media player has one, and if it doesn't I'd suggest using foobar2000 or Winamp. Both are excellent players, and in the opinion of the majority on this forum a lot better than e.g. iTunes or Windows Media Player.

Other than that you might also need a DAC. And for $400 it might be best to get a Amp/DAC combo. However, that all depends on your soundcard. Could you state the exact model? Because if the soundcard has a good enough DAC then it's not necessary to buy a separate one.
In the case you don't need a DAC, I'd definitely check out the Little Dot MKIV, or MKIV SE. I have the MKIV SE and it's a beautiful amp that definitely does my HD 650 justice.

That brings me to my final point: are you looking for solid state or tube amplifiers?
Solid states are generally more precise and slightly cheaper for the same fidelity.
Tubes are less precise and sound a bit warmer. They also allow to change the sound signature by tube rolling, whereas solid state amps can rarely be changed (without desoldering op-amps)