I stand corrected. I was using a minidisc recorder at the time, so using the y to stereo mini cable.
Even so, unless the PMD661 has faulty preamps the overall self-noise improvement from upgrading to a SD preamp is going to be marginal. The Avisoft self-noise tests include a link to a page titled
http://www.avisoft.com/tutorial_mic_recorder.htm, which discusses what I was trying to show with the calcs above. The illustration in particular shows the relationship mic self-noise and sensitivity vs recorder EIN.
Where the recorder EIN really becomes important for ambient recording is when the mics have low output noise. The AT3032 and it's replacement the AT4022 are pretty popular mic for nature recording, and the AT3032 was independently tested as having a self-noise of around 8dB(A) and a sensitivity of 20mv/Pa. This gives an output noise of -117.8dBu(A) which makes it a fairly demanding mic in terms of preamp self-noise, and these mics require a recorder with an EIN -126dBu(A) or better.
cheers
Paul