Airpot Express - Upgrade amp or DAC?

This is not accurate. DAC will improve AE since it has relatively low jitter on digital output but horrible jitter artifacts on analog out. You can find measurements here:

http://www.stereophile.com/digitalprocessors/505apple

Any jitter suppression, either by reclocker or upsampling (reclocking) DAC is always beneficial.

Yes, I use Benchmark DAC1 with AE and sound is incredibly clean up to point of being too clean. You might not like sound of the Benchmark DAC1 or any upsampling/oversampling DACs but cannot deny its jitter suppressing ability. Measurements taken by few reviewers, including Stereophile, confirm it. In fact all the measurements of Benchmark DAC1 are phenomenal.

http://www.posthorn.com/Bench_5.html

Steve, I'm aware of changes you made to DAC1. These changes, as I remember eliminated jitter reduction ability of DAC1.

Effects of jitter are not even detectable buried deep in -130dB noise floor according to many different reviews. PLL on DAC1 is used only for the syncing while main reduction of the jitter is obtain by reclocking in asynchronous sample rate converter. Perhaps I don't have as good hearing as you but my Benchmark sounds identical with different sources. It sounds a little better with different op-amps (tried few) but even original NE5532 (TI) sounds pretty good, unless you had earlier version of this amp (thin sounding) made by Signetics before their factory burned down in 2001. TI version has different (larger) die.

Original question was about choice between upgrading amp or DAC. Choice is quite easy since AE has 2400ns peak to peak jitter on analog output and respectable 258ps peak to peak jitter on digital out.

According to Benchmark Media technical director John Siau DAC1 was not designed to sound warm but natural. He stated that warm sound (enhanced even harmonics) does nasty job on instruments (like piano or percussion) with harmonic structure more complex than simple overtones. Piano might even sound, on overly warm gear, like out of tune.

Benchmark DAC1 is very clean and revealing making many systems sound bright. That was the case with my previous speakers that had aluminum dome tweeter. Hyperion HPS-938 I use now are perfect match for the Benchmark and Rowland class D amp. Not only that it sounds perfect to me with extremely clean sibilants and amazing details but also because of AE I don't have to play games with expensive playback programs, computer speed, amount of RAM memory etc.

Steve, Isn't Metrum Octave a NOS DAC? I would need reclocker for best results with AE. Many people go for NOS claiming ill effects of digital filtering (pulse response) but reclocker most likely involves digital filtering defeating purpose of NOS. Did you hear anything about Benchmark DAC2 HGC?

"Then why can I easily hear these differences? Why do other reviewers hear them?"

I don't know. It could be many reasons starting from not bit perfect sources, playback programs that do processing of the data, ground loops etc. or it is possible that my hearing is not that good or I am under placebo effect after seeing FFT plots. I've read reviews claiming audible difference but also read many reviews being unable to detect the difference. One thing for sure - almost everybody praised the clarity, some even calling it "ultra clean" what wouldn't be the case with jitter present.

Thanks, Steve. I'm committed to wireless network but will wait few years to make big step with new DAC (and possibly reclocker). It looks like D/A chips improved but also reclocking chips and analog electronics are making big steps.

Steve, who makes reclocker with this new TI chip?

Thanks Steve. It looks interesting.