Hey,
Just wanted to give my 2 cents worth...I have the same problem on a 2008.5 Mazda3 Sport, single CD, non-Bose. I took it into my dealership about a week and half ago, told them the issues and they couldn't verify themselves and nobody had heard of this issue before. Just this morning I took it into a different dealer/service dept. whom I had called beforehand. Over the phone the service manager said he had just received the service bulletin and what I was telling him matched the issues.
The technician this morning could not verify the issue (though I think my explanation was a little unclear - they thought I meant at speeds around 34-35 instead of VOLUME 34-35) so I went on a drive with him to show him. He was not really hearing what I was though I could get him to acknowledge the huge sound increase/change from 34-35. The whole bass dropout issue, at least on my car, seems pretty random and even though I was able to hear it, the technician was not. When we brought my car back the service people were a little reluctant to verify the "issue" but decided to pull the unit and run the numbers and all that to see if my unit fell under the service bulletin. After about 15 minutes they let me know that they would be ordering me a new unit, yes! 3-5 days was the estimate.
Here's some info - I saw the service bulletin for a minute but was not able to get a # off of it or a copy. I noticed that there was a VIN # range and a manufacture date range of Jan. XX, 2008 - March 20something, 2008. The service bulletin applies to all 2008.5 Mazda3 and Speed3 cars and all stereo systems, Bose or non-Bose with ALC on or off. They have to pull the unit out and get what I am assuming was a serial # and then match it with certain software codes or something like that. It's definitely a software issue with the head unit. Advice: When you bring it in be very clear about the issue(s) and turn up the bass before they test it - a technician has to verify before they will even pull the unit to figure out how to proceed.