[Utopia]

Another option might be the Matrix M-Stage (with or without built-in DAC). 

 

[inthere]

Keep the stack and get Fostex TH600's, which have had a 40% price cut and are now $599. They go perfectly with that stack for the sound you're looking for, very smooth and warm, but with loads of clarity too.

 

[Mkilci]

I have an HD650. Tried the O2 (Amp only, through a SB E5 DAC). Was not impressed at all and returned it. I also got the Brovo Audio Ocean. Love the Ocean with the HD 650. (Please keep in mind I am a total audio noob and have very little experience.) Going by purely what I hear...

 

[Dickies]

Have you given EQ a try? I think you should try before getting different gear
I found I couldn't get into my hd 700's until I found a eq setting
firstly to tame the spike to my ears I    -8db at 5900hz  4q
it's less important but I also bosst 2500hz 2bd 4q  and +3db at 35hz 1q (I'm not a bass head, if you are use a bigger boost) 

 

[cel4145]

As far as frequency response goes my tough to drive headphones are the k701s and they will sound thin when under powered.

Yes. I have the K612 Pros, which have a lower sensitivity rating than the K701s. The HD700s have a higher sensitivity than both of those headphones.

Damping examples:
- For example, a 32 Ω headphone driven by a headphone amp with a <1 Ω output impedance would have a damping factor of >32, whereas the same headphone driven with an iPod Touch 3G (7 Ω output impedance) would have a damping factor of just 4.6.

The HD700s are 150 ohms. There's no issue with the dampening factor.

 

[Music Alchemist]

 

I'm not even sure I could sell the HD700 nor the O2+ODAC (I don't live in the states).

 
You can list them on the FS forums and ship to anywhere in the world. Just charge the buyer whatever the shipping cost is.
 

I guess the combination of the HD600/650 and the Bottlehead Crack could be the best for me? It's not like a downgrade, right? (from HD700 to HD600/650...)

 
I like the HD 650 a lot more than the HD 700. Despite being darker, it sounds more coherent.
 

And btw, don't I need a DAC? What I have now (O2+ODAC) is a combo of amp and DAC, but the Bottlehead Crack is only a tube amp (I'm pretty sure the on-board DAC I have in my motherboard is horrible?)

 
I would recommend comparing with and without a DAC to see if you can hear a difference.

 

[NuClear235]

I have iDSD Micro with Sennheiser HD800. I love this combination. ODAC was not good for HD800.
There is possible to buy cheaper version iDSD Nano with the same DAC. Maybe is this way for You...

 

[Rado2]

I have the same combo and I do like it. Prevailing is - yes, tube amps or EQ settings for some listeners.
As for the power: O2 on 1-to-1 gain will bring the cans near 112 dB (If I remember correctly) - so enough power. I rarely boost past the half of the volume knob on this gain for example. It will be interesting to know what you did at the end?

 

[kgong]

try the akg k712