I own the 15.32 but I have used the Matrix M stage I have not used the HRT music streamer 2 tho, but Both M-Stage and the NFB15.32 headphone amp is very strong and they be able to drive most headphones that req alot of power. The good thing about the NFB15.32 is that its all one unit so you don't take up much room, It does have digital filters which you can change the sound signature, only a few of the settings i can tell the difference between, while the rest I can't. The Default Digital filter is a tad warm, but the Digital filter I using now is hard to explain. Now the Matrix M-Stage Default op-amp I did not like so I changed it out depending on which headphones I was using with it. The M-Stage has room around the op-amp socket for using op-amps that are on a dual to single adapter which is common used for using two single channel op-amps or cap's. The M-stage if I recall gets very hot, as well as the NFB does get hot as well both make perfectly good heaters when winter comes. II recall that the HRT streamer 2 has a op-amp socket,from pictures of the inside.
The good thing about the USB input on the NFB15.32 most people don't know this, is that the NFB 15.32 supports Full blu ray audio in it glory meaning you wont get down-mixed blu ray audio when your using the USB input thx to the USB32 transfer chip, it works as I tried a couple of my blu ray movies. I only have minor issues with the USB issues but i think that relate to my motherboard usb issues that I have and not the device it self. There are M-Stage models that come with dac's inside of them but they cost more and I don't know how good it would sound. I would say just get the cheaper of the two option's or if you lack room get the one or combo that take up less space.