See if you adjust to it over time. I felt similarly about this line despite them having a fairly flat-ish response down to 20Hz (does roll off a bit in the sub-bass). Part of it was just that I needed adjustment coming from bass-heavier headphones. The HD545 may have had more bass and/or mid-bass. Alternatively, the HD545 may have actually had higher harmonic distortion in the bass, which can make headphones sound "fuller." (Makes sense if you think about it...say you play a 60Hz tone, with high harmonic distortion, there may be audible spikes that also occur at 120, 180, 240Hz, and so on, which can change how one perceives the bass response.)
It could also be the amp you're running for these. They are fairly sensitive and will run off about anything, but they do scale with better amps and will sound better controlled and fuller that way.
The HD600 is more expensive and much harder to driver properly, but it sounds noticeably fuller and cleaner than the HD5x8 line despite having a fairly similar frequency response and overall lower bass distortion. It is what it is.
You can also try EQing the ~5KHz spot down on the HD558 (or, more broadly, the 3-6KHz area with a peak centered around 5KHz). It has a bit of a peak there that could potentially make them sound leaner and sharper. There is also a simple mod for them (foam removal, reversible) that could slightly alter their sound more towards your tastes.
Similarly, I have some open-cell foam material (thin sheet) you could experiment with placing in front of the drivers. That will boost the mid-bass response and slightly tone down the response above 1KHz, giving them a fuller, warmer sound.
Feel free to PM me if you have any other questions or want other suggestions. And stay away from Head-Fi...really messed up community with a lot of crazies (every headphone is the best headphone!).
Edit: Oh, manufacturer specs for headphones are basically meaningless. Measurements than analyze the frequency response, distortion, spectral decay, impulse response, and so on from trustworthy sources will give you a better idea of a headphone's performance, though they don't always explain everything perfectly.
