How to Match Levels?

I want to do some testing of digital sources with my Stax Omega phones, but I have no idea how to properly match the levels to within the golden .1db goal. I have heard that a sound meter (a la Radioshack) won't cut it.. why not? If I place the phones properly and place the mic between the phones and play 1 KHZ tone or something, how close will I be getting to properly matched levels. After years of buying gear based on sound and opinions, I honestly want to test some things now, and am surprised at how little info is available...

Any help?

Presumably, you have a mono switch in your system, probably at the preamp. If you use a sound level meter in the middle between your headphone speakers, you would play mono and adjust the balance to a maximum on the meter. I have the Radio Shack analog sound level meter, and I don't think it is precise enough to work with headphones within 0.1 db.

In my opinion, the best way is to put on the headphones, play a mono signal, and adjust the balance until the sound appears to come to you exactly in the middle of your forehead. This has the added advantage of centering the signal for any differential in your ears' sensitivity, as well as the headphone speakers. It may not be the ultimate in precision, but any imprecision is below your level of detectability anyhow. Good luck.
Is it possible to unplug the wire(s) at the headphones?-- as it is w/ Senn. 580/600s? If so, you might try doing that. If not, find some other way to mute the channels individually. Of course placing the SPL meter will be critical for determining dB output for each side. It seems to me that the RS SPL meter (the analog one) is only readable to about the nearest 1/2 dB though. Does their digital meter read in tenths?

I've never tried to do what you suggest, but the Sennheiser cords do unplug. Do the Stax have some way to adjust level for each side? Good Luck. Craig
Are you trying to match levels at the headphones or at the source feeding the amplification chain ? Sean
I seem to recall that the specs for the Radio Shack SPL meters say they are accurate to 1 or 2 dB (even though they may read in finer gradations). That should make you think twice about using them for level matching. They simply aren't designed for that use. Wrong tool for the job.

Seems to me Sean's got the right approach: level-match with a voltmeter somewhere earlier in the chain. Gain will be the same for all sources from that point on, so you can safely assume that what you're actually hearing is matched.
Well, I am ideally trying to match the levels on the source devices (cd players, dacs). But I have no way of attenuating their signals so they are putting out the same voltage. All I want to do is to be able to A/B source components at the same levels. However, the pin config for Stax phones is funky, and has some very high voltages coming through it..

So, if I simply want to A/B source components and get the volumes equal.. what do I do?
The SLM idea won't work. You need to run the sources into a preamp and monitor the output of the preamp with a volt meter using a 1K test tone. The volume control on the preamp can be used to match the levels. This is a drastically more accurate method than the alternatives.