i have. long story short i don't think any SS amp can beat the Beta22 (as far as my own tastes go), but the Apache does offer the RSA house sound just like the SR71 portable amp. it has a thick, palpable midrange that can be pleasing depending on the music.
in all the amps i've heard, balanced sounded more transparent than single-ended. the 3-channel active ground does lower output impedance, but does nothing for slew rate. full balanced is the way to go.
M^3 is warmer. the Dynahi and Beta22 are even warmer. the CK2III is also a very good amp and very easy to build, but yeah the M^3 is warmer. i prefer the CK2III personally.
all the FET-output amps i've heard are quite warm, while the BJT-output amps can go either way.
3dB or 6dB of attenuation is not enough and will only give very slightly more play on the volume knob. You need maybe 10-20dB of attenuation if 9 o'clock is already super ultra loud. This works very well, and cheap (just a couple resistors):
http://goldpt.com/mods.html (under Pre-Attenuation)