Great stuff, and an added incentive for me to get the Rives disc since I'm going to embark on an upgrade of my DIY speakers within the year.
One possible way around the 1/3 octave limitation on the Rives disc is to get a disc with low-frequency sinewave sweeps, say up to 200Hz or so.
The Linkwitz test CD has such sweeps, plus tonebursts at various frequencies and pink noise, useful for other room interaction and speaker tests. You can watch the SPL meter wiggle (or the readout change) as the sweep plays. You won't be able to exactly know the frequency at which a given null or peak occurs, though, without some sort of frequency counter. Some DMM's (digital multimeters) have this feature at a not too outlandish cost (and there's always eBay). Or, you could use a stopwatch and some math to find the approximate frequency since the sweeps are of a specific duration, but...
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Or you could go on eBay and find an old cheap frequency generator like I did.
There are also shareware spectrum analyzers and frequency generators for Mac and PC out there at reasonable prices.
Of course, none of these offers the Radio Shack meter calibration of the Rives disc but they can still be useful additions to it.
That result you got looks awesome!
Oh, and I second Doc's idea of testing affordable EQ units like the Behringer and others! Not just for THD+N (that's important of course) but overall performance.