Does Tablo Ripper actually pull a show bit for bit in the format Tablo records, or does it do its own encoding? I suspect it does the latter, in which case I would think that the 255kbps AAC stream is a re-encode of the actual Tablo audio stream. That would be just like taking an MP3 that is 64kbps and converting it to 256kbps AAC-- it would still sound like a 64kbps MP3, since all of this is lossy compression and you can’t get back data that was already discarded by the first compression.
I wish the folks from Tablo support would tell us the audio compression scheme that the box uses (format, bitrate, sampling rate, etc.) as that would confirm my suspicions that they are using a low bitrate and/or sampling rate. This might be a limitation of the chip that does the audio encoding, or perhaps is a setting that they could change via firmware update.
That being said, I will give Tablo Ripper a try and see what the quality on the computer is like. I find it odd that a few others are having similar come,aunts about the audio quality. Perhaps the difference in playback equipment means some folks notice the compression more than others?