yup XM5 is still available but yes, there is an upgraded version, the XM6(yeah, who-da-thunk). The XM6 has a better DAC section but very similar amp section. I have not heard the XM6.
I have used the XM5 with the HD-650. As you might know, the 650 will impress with a lesser amp but will really shine with a powerhouse. One thing that I always tend to comment about re the HD-650 is its ability to play super-loud while remaining "clean", just capable of dishing out excellent dynamic 'slam'.
So no, the XM5 couldn't match my Woo Audio 3 tube amp for absolute dynamic goodness but it goes many levels above what any DAP (aka:mp3 player) can muster.
The only portable amp that I've used that surpassed the XM5 for ultimate power output was the Ray Samuels SR-71A which uses two 9-volt batteries compared to the single 9-volt with the XM5.
My XM5 has buffer chips(TI BUF634) which bump up the power and I've also changed the stock op-amps to the AD8065 to further enhance its kick-ass ability.
http://www.practicaldevices.com/
Considering that I've owned quite a few portable amps, the XM5 remains with me. Even though the SR-71A amp had more power, it was a featureless amp. The XM5 has a USB DAC, A very nice adjustable cross-feed, Bass and treble boost ( +6 dB. @ 100Hz. , +3 dB. @ 10kHz ), gain switch, +75 Ohm switch (great for Etymotic ER4P). It also has a battery level readout in tenth of volts by way of LED lights in addition to a selectable timer for switching off to conserve battery power. I have the power supply option that allows a lithium battery to be charged inside the amp.
-yeah, a lot of cool stuff packed into a small box...and that's why I still have this amp.