What kind of DAC/AMP would I need?

Buying a nice soundcard could help. The 555's are easy to drive. 

Just my 2 cents. 

The uDAC2 and Audinst HUD-mx1 are nice little plug-n-play USB DAC/amps for under $200.

The advantages of the E7 are that it is portable, and it can be docked into the E9 desktop amp. Actually, I believe Micca Store sells E7+E9 together for $200.

But putting portability aside, and if you don't intend to get the E9, then the uDAC2 or Audinst look like better options. The uDAC2 and Audinst both support up to 24-bit/96k over USB, while the E7 does not. (Though it's not really that big a deal since CD quality is only 16-bit/44k anyway.) And for me personally, the analog volume knob is a big plus.

Here's a similar comparison that I dug up:

http://www.head-fi.org/forum/thread/519471/udac-2-vs-fiio-e7-vs

Quote:

Originally Posted by justanotheraudiophile 

I would prefer an amp/dac that lets me take advantage of the high-bitrate tracks that I have (I dont intend on producing any audio) although I am not sure how most of the songs available for download stand in comparison to the "standard cd quality" you speak of, I don't know much about bitrates..and I assume bitrates can be falsified? 

 

For any music download that originally came from a CD, the highest quality it can be is 16bit/44k lossless. It can be compressed into MP3 with various bit rate levels (128k, 192k, 320k, etc.) but those numbers are a separate thing. Some websites like HDtracks offer downloads that are better than CD quality. These downloads are sourced from SACD or DVD-Audio. So they can have be 24bit/88k, 24bit/96k or even 24bit/192k. And you are correct, it is possible to falsify bit rates by up-converting a normal CD track (or even an MP3!) into a hi-res 24bit/96k file. But with proper analysis using an audio editor, you can tell the difference by visually looking at the wave forms.

But again, I have not been able to tell the difference between 44k and 96k while listening normally. And I don't have the patience to listen critically enough for that. Quite a few people swear that they can ear the improvements though. 

The uDAC2-HP is the "lite" version of the full uDAC2. No RCA outputs (for speakers or separate amp), and no coaxial digital output (for separate DAC). Also, the volume knob of the HP may have channel imbalance at lower volumes. The EMU series is quite well respected as a pro audio device. You get recording capability as a bonus. I'm not keen on needing to install drivers for it, but that's just me.

Here are the power ratings of the respective headphone amp sections:

EMU 0204: 16mW (into 22? Ω)

uDAC2:  80mW (into 16 Ω)

E7: 150 mW (into 16 Ω)

Audinst HUD-mx1: 800mW (into 32 Ω)

The Audinst clearly has the most powerful amp, but my guess is that any of these will do fine with the HD555.

The uDAC (if you count it since version 1) has been FOTM here for as long as I have been on Head-Fi.

Yep, get the full uDAC2 if you want the RCA output. Note that the volume knob controls both the headphone out and the RCA out. And the RCA output is muted when you plug in headphones to the uDAC2.

Right. But if you want to go with the adapter solution I suggest using an RCA splitter at the uDAC. One goes to the sub, and the other to your headphones. Actually, you could also do it the other way and split the headphone output instead. Plugging headphones into the sub is not recommended.

You know, now that the sub has come into the picture, and with the possibility of you going portable, it might be worth looking into the E7+E9 combo. I know that the E9 has both a fixed volume  line-out as well as a controllable preamp out. What I don't know is whether both are muted when you plug in headphones.

You can ask about the E9 in this thread:

http://www.head-fi.org/forum/thread/518997/the-official-fiio-e9-thread-with-video-unbox-review

There's a good deal now on the EMU.

http://www.head-fi.org/forum/thread/532704/e-mu-0204-usb-damn-they-ve-done-it-again-and-for-129/105#post_7294988

I believe the 0204 can output to both headphones and line out at the same time.

Sorry if that's not helping you narrow down your choice. But like I said they should all do fine. Just pick based on what features you need.