In the tests I have done with the PRC-320 so far, I have got repports of weird modulation as if the audio sounded tinny.
I have consulted this issue with the fellows at the hfpack yahoo group and I got very interesting replies. I will quote a brief extract of the reply by Mark Francis (KIoPF), who is an authority in military radio equipment:
The human male voice develops audio energy from nearly zero cps in the bass to way up there in the multi-thousand cps region in the sibilants ("S"-sounds).
But audio engineers long ago determined that the most important range for effective communication was around 300 - 3000 cps. The initial research was done for telephone lines, but it was quickly found that the same specs were good for AM voice transmission, then SSB.
Furthermore, it has been demonstrated over and over that in really noisy, bad propo conditions on the HF bands (like what we have now, although it's getting better), pretty much any audio energy developed below 500-600 cps is wasted - it gets lost in the noise.
On a packset you have two big strikes against you to start with -- low power and often marginal antenna systems. You need every advantage you can get! Modern military packset designers have realized this and have usually designed their audio to really light up in the 600-3000cps region, then on top of that have included heavy-duty speech processing to enhance the mids and highs even more.
You PRC-2000 owners will know what I'm talking about. That set has the best audio for cutting through noise I've ever heard. But it'll drive the so-called "hi-fi SSBers" absolutely bonkers.
On my PRC-1099s I know to be working to spec, I've had people tell me my filters are bad, the audio is kaput, or I'm off-frequency. This last is because they erroneously interpret the "tinny" sound, as you call it, to the radio being off-frequency, instead of actually listening for the real indications of an off-freek SSB signal.
Mark Francis (KI0PF) is the author of Mil Spec Radio Gear Volume 2 available direct from the author (signed, on request) and from Universal Radio, Fair Radio and Electric Radio. $29 cash/check/M.O. or $30.17 via Paypal, when ordereddirect from KI0PF & shipped to the U.S. (foreign buyers please contact him). See: www.mmfrancis.com/MSRG2/MSRG2.pdf. Volume 1 is still available from CQ, Universal Radio, and Amazon.