Upgrade from entry level

the lack of satisfying bass can likely be attributed to the hd555.

i owned the hd595 for a while (which is basically the same headphone with slightly different housing), and found the bass to be overly polite.

if you like the sound of the hd555 otherwise, it may make sense to go with the hd650, which is generally considered to be a warm headphone overall.

the grado sr325is can be a gamble.  some folks love it.  other despise it for the treble energy.  so be warned.

from what i have read, the 0404usb is pretty good, and should allow you to scale up to the mid-range audiophile cans without being a weak link (think k701, hd650, dt880, ad2000, etc.).

someone correct me here if i'm wrong.

good luck with your upgrade!

yes, a dedicated headphone amp is pretty much requisite for moving up to some of the nicer cans.

The 0404 USB's headphone output can be a weak link in certain scenarios. While its output is clean most of the time, it has a fairly high output impedance and a limited amount of drive available according to EMU's specs:

Headphone Amplifier

			Type: Class-A power amplifier
			D/A converter: AK4396 (shared with Line Out)
			Gain Range: 60dB
			Maximum Output Power: 20mW
			Output Impedance: 22ohms
			Frequency Response (20Hz-20kHz): +0.06/-0.035dB
			Dynamic Range (A-weighted): 114dB
			Signal-to-Noise Ratio (A-weighted): 114dB
			THD+N (1kHz, max gain): 600ohm load: -95.5dB (.0018%)
			Stereo Crosstalk (1kHz at -1dBFS, 600 ohm load): < -85dB

According to the above, max output power is only 20mW - I find this a bit hard to believe, but hey. The output impedance is 22ohms so headphones with wildly varying impedance curves will be a bit succeptible to the voltage divider effect.  I can confirm that the 0404 works fairly well with the host of usual mid-range headphones in my collection: K702, HD650, DT990/DT880, MS1, etc.

I am looking to augment the 0404 with a dedicated desktop amp, using the line out signal.  I hope to get better results and more drive volume this way.  One thing to be careful with the 0404 is that its line out at max is about 16dB hotter than the typical consumer grade equipment. Setting the line out signal to the max will sometimes over drive the input stages of headphone amps. The output has to be dialed back a bit to ensure that this doesn't happen.

Jack

I would say it's the headphones to blame first, then the 0404 amp, and last of all the 0404 DAC. The DAC in the 0404 is very unlikely to be contributing to your lack of bass.The DAC in the 0404 is very highly rated. The 0404 amp on the other hand will be underpowered for hard to drive phones. Look to the phones and 0404 amp before you even consider upgrading the DAC.

Transducers always make the biggest differences, you have to get that, then you get the amp, but only to support the transducers. The reality is that rock and pop don't take too kindly to bass quantity, it tends to cloud up the music, usually with a lot of the crappier volume war recordings out there, electronica does better on the whole. My instincts tell me to recommend a RS1 straight from the 0404, but it's not a reliable opinion since I haven't head the Grado in a few years. 

Yes.