I batched half a dozen albums with both settings and the files size are identical.
Files size are almost identical. Compression gain is very small: 0.11%.
For example,
FLAC 8 -Ax2 - 400 MB
FLAC 8 - 400.44 MB
OK. I've had some more time to do some more tests. I encoded 3 more albums from wav to flac, with and without -Ax2. I'm still getting identical results. I purposely chose some quiet albums to highlight better compression.
Flac -8 Log
flac -8 -Ax2 *.wav
flac 1.2.1, Copyright (C) 2000,2001,2002,2003,2004,2005,2006,2007 Josh Coalson
flac comes with ABSOLUTELY NO WARRANTY. This is free software, and you are
welcome to redistribute it under certain conditions. Type `flac' for details.
01 Ophelia.wav: wrote 24636257 bytes, ratio=0.451
02 Life Is Sweet.wav: wrote 33682625 bytes, ratio=0.615
03 Kind & Generous.wav: wrote 23922555 bytes, ratio=0.548
04 Frozen Charlotte.wav: wrote 26079007 bytes, ratio=0.458
05 My Skin.wav: wrote 30572115 bytes, ratio=0.525
06 Break Your Heart.wav: wrote 28175170 bytes, ratio=0.556
07 King of May.wav: wrote 25433586 bytes, ratio=0.578
08 Thick as Thieves.wav: wrote 41283485 bytes, ratio=0.562
09 Effigy.wav: wrote 11950741 bytes, ratio=0.451
10 The Living.wav: wrote 16735856 bytes, ratio=0.483
11 When They Ring the Golden Bells.wav: wrote 49785363 bytes, ratio=0.492
And the flac -8 log:
flac -8 *.wav
flac 1.2.1, Copyright (C) 2000,2001,2002,2003,2004,2005,2006,2007 Josh Coalson
flac comes with ABSOLUTELY NO WARRANTY. This is free software, and you are
welcome to redistribute it under certain conditions. Type `flac' for details.
01 Ophelia.wav: wrote 24636257 bytes, ratio=0.451
02 Life Is Sweet.wav: wrote 33682625 bytes, ratio=0.615
03 Kind & Generous.wav: wrote 23922555 bytes, ratio=0.548
04 Frozen Charlotte.wav: wrote 26079007 bytes, ratio=0.458
05 My Skin.wav: wrote 30572115 bytes, ratio=0.525
06 Break Your Heart.wav: wrote 28175170 bytes, ratio=0.556
07 King of May.wav: wrote 25433586 bytes, ratio=0.578
08 Thick as Thieves.wav: wrote 41283485 bytes, ratio=0.562
09 Effigy.wav: wrote 11950741 bytes, ratio=0.451
10 The Living.wav: wrote 16735856 bytes, ratio=0.483
11 When They Ring the Golden Bells.wav: wrote 49785363 bytes, ratio=0.492
Finally, I ran Flake -8. According to the Flake Website it should compress higher. But my results show the opposite:
flake -8 *.wav
Flake: FLAC audio encoder
version 0.11
(c) 2006-2007 Justin Ruggles
block time: 105ms
variable block size: none
prediction type: levinson-durbin
prediction order: 1,12
partition order: 0,8
order method: 4-level
stereo method: mid-side
header padding: 4096
input file: "01 Ophelia.wav"
output file: "01 Ophelia.flac"
Signed 16-bit 44100 Hz stereo
samples: 13651008 (5m9.546s)
block size: 4608
progress: 100% | ratio: 0.452 | bitrate: 637.4 kbps | bytes: 24636971
input file: "02 Life Is Sweet.wav"
output file: "02 Life Is Sweet.flac"
Signed 16-bit 44100 Hz stereo
samples: 13690404 (5m10.440s)
block size: 4608
progress: 100% | ratio: 0.617 | bitrate: 870.7 kbps | bytes: 33753494
input file: "03 Kind & Generous.wav"
output file: "03 Kind & Generous.flac"
Signed 16-bit 44100 Hz stereo
samples: 10916220 (4m7.533s)
block size: 4608
progress: 100% | ratio: 0.550 | bitrate: 776.8 kbps | bytes: 24006196
input file: "04 Frozen Charlotte.wav"
output file: "04 Frozen Charlotte.flac"
Signed 16-bit 44100 Hz stereo
samples: 14242536 (5m22.960s)
block size: 4608
progress: 100% | ratio: 0.458 | bitrate: 646.1 kbps | bytes: 26047212
input file: "05 My Skin.wav"
output file: "05 My Skin.flac"
Signed 16-bit 44100 Hz stereo
samples: 14560644 (5m30.173s)
block size: 4608
progress: 100% | ratio: 0.526 | bitrate: 742.8 kbps | bytes: 30649617
input file: "06 Break Your Heart.wav"
output file: "06 Break Your Heart.flac"
Signed 16-bit 44100 Hz stereo
samples: 12670224 (4m47.306s)
block size: 4608
progress: 100% | ratio: 0.559 | bitrate: 788.8 kbps | bytes: 28306993
input file: "07 King of May.wav"
output file: "07 King of May.flac"
Signed 16-bit 44100 Hz stereo
samples: 10994424 (4m9.306s)
block size: 4608
progress: 100% | ratio: 0.579 | bitrate: 817.8 kbps | bytes: 25472016
input file: "08 Thick as Thieves.wav"
output file: "08 Thick as Thieves.flac"
Signed 16-bit 44100 Hz stereo
samples: 18359124 (6m56.306s)
block size: 4608
progress: 100% | ratio: 0.564 | bitrate: 795.3 kbps | bytes: 41372650
input file: "09 Effigy.wav"
output file: "09 Effigy.flac"
Signed 16-bit 44100 Hz stereo
samples: 6622644 (2m30.173s)
block size: 4608
progress: 100% | ratio: 0.448 | bitrate: 632.1 kbps | bytes: 11861876
input file: "10 The Living.wav"
output file: "10 The Living.flac"
Signed 16-bit 44100 Hz stereo
samples: 8668884 (3m16.573s)
block size: 4608
progress: 100% | ratio: 0.483 | bitrate: 682.2 kbps | bytes: 16740442
input file: "11 When They Ring the Golden Bells.wav"
output file: "11 When They Ring the Golden Bells.flac"
Signed 16-bit 44100 Hz stereo
samples: 25276356 (9m33.160s)
block size: 4608
progress: 100% | ratio: 0.494 | bitrate: 697.0 kbps | bytes: 49924335
Summary. flac -8 and flac-8 -Ax2 are identical size and flake -8 is larger (but should be smaller). I have repeated this several times with different albums with the same result. Can somebody else run a similar test and produce different results?
On a side note, is flake -8 the highest setting which is compliant with the flac reference encoder.
Thanks guys.