I also had a D2 Boa, and the XXS and T4 trumps it IMO.
The D2 is both a headphone amp and DAC, so it takes either input from USB when connected to computer (ie using the DAC function) or just a normal 3.5mm input (ie using it as a headphone amp). T4 and XXS do not take USB inputs, thus are not DACs and are only normal headphone amps.
If you plan on listening from computer all the time, then why not consider getting a proper desktop dac/amp combo like a Zero? Much better performance for the same amount of money (Provided you don't need to lug it around).
http://www.head-fi.org/forums/f5/rev...re-amp-269458/
Don't worry about the XXS charging. Just get a single rechargeable 9V battery and keep using it (I got a 280mha one from dealextreme). The battery lasts ages on my XXS. T4's battery life is also rather good (Still haven't drained the battery yet, so no firm figure).
Send Jan an email to order a headsix. There's a thread dedicated to this amp as well.
I have no idea if you can DIY, but another suggestion I'm going to throw in is the mini3. Fantastic sounding amplifier, and you can DIY one for less than the cost of the T4/headsix. Plenty of help online if you run into trouble, and there are full kits out there as well.
What headphone/IEM are you planning on using the amp with? 50ohm doesn't really tell too much....