Which headphones??

I can't stress enough the need to try each and any headphone you plan on using as they are very particular and unique to each and every set of ears. No two will sound alike to any two people you care to mention. Listener bias is the only deciding factor.
Nonoise

Agree with this. Another reason to audition is comfort. I have never been able to use over the ear phones for any length of time, max out at about 30 mins. What does work for me are in ear monitors. Their sound can be quite good also.

Thanks everyone! That was the second time beyer T70 came up. I'll look into those more, and also some HiFiMans, and sennheiser 600/650's.

Next question; how do you tell a headphones "sensitivity" rating? Lower ohm's more sensitive?

How do you tell a headphones "sensitivity" rating?

Sometimes it is explicitly specified, as sound pressure level (in db) for a 1 volt input.

In other cases it may be specified as SPL for an input of 1 mw (1 milliwatt, which is 0.001 watts). To convert that to voltage sensitivity use the equation

P = (Vsquared)/R

Where P is power, in watts.
V is voltage, in volts.
R is impedance, in ohms.

For example the T70 is spec'd at 250 ohms, 104 db/mw.

The voltage required to generate 104 db would be:

V = Square root of (0.001 x 250) = 0.5 volts.

To calculate the SPL that would be produced in response to a 1 volt input, so that you can compare to other phones which are spec'd based on a 1 volt input:

1 volt/0.5 volts = 2

db = 20 x (logarithm of the ratio of two voltages) = 20log2 = 6 db

104 + 6 = 110 db SPL for a 1 volt input

Best regards,
-- Al

Lol, so that's what it sounds like to my fiance when I talk to her about stereos or cars ;-)

Step up to the HD580/600 and you'll probably be much happier long-term, as they're much more sonically refined than the lower-tier Sennheiser models. You can pick up a used pair of HD580 (which I own now) for under $150 and the HD600 (which I used to own and should have kept: they're definitely a touch more transparent) for around or just above $200.

For context, I owned the HD555 modded to HD595 (model prior to the HD598), and the HD580/600 series is a huge step up. The lower models have a mid-bass bump and some grainy treble emphasis, but the 580/600 sounds smooth and neutral top to bottom (though some find them too warm).

Here's a great recent comparison of the HD580/600/650 series:

http://www.innerfidelity.com/content/very-important-sennheiser-hd-580-hd-600-and-hd-650