Which headphones??

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

regardless of specifications and performance, headphones are way more individual sensitive than speakers.convenience, head size, shape, earshells it all matters to get a perfect headset. i get headaches and listening fatigue from overdefined HD580/HD600 bass. did not try hd650 as an example.
your best bet is either swapping them one after another by purchasing/selling or visit a good headphone shop or even audio festival to make up your decision.

Marakanetz, Are you satisfied that you used an amp that best matched the Senn's you listened to. FWIW, for example, I tried a highly regarded amp, as well as a CDP with a capable built in amp. Both of my Senn Headphones (598 and 650) sounded like crap with SS amps. Especially the 650's.

FWIW.