^REgarding difference in SQ between 105 and HA1...
Rich seems to hear a moderate difference in favor of the HA1.
Personally I also hear a difference, which I consider slight but noteworthy in favor of the HA1.
Eljr says no difference at all.
So I do think we can determine that it is dependent on a particular set of ears and a particular system.
I think we can all agree that the difference is not night and day.
However the bigger differences are the feature sets of the 105/105D vs HA1...
They have very different features that any given person may find more useful in one compared to the other.
The HA1 can handle DSD128 and DSD256 via the async usb inout, has high gain option, analog audio inputs, Bluetooth,
and my personal favorite where you can connect an iPhone/iPod/iPad directly to the front USB input, and probably a couple other things I am forgetting right now.
If any of those are features that you think is worth the price of the HA1 after already owning a 105, then it's a personal choice you'd have to make.
If you are budget conscious and happy with your 105 and it's headphone amp / DAC / preamp options, then it may be better to stay with that.
If you don't need the additional features of the HA1, then again you may be better off staying with the 105.
One thing is for sure though. The HA1 is one awesome piece of equipment.
Anyone who does decide to buy one will not be disappointed.