I'm not an apple computer fan so I will not comment on your choice of source, but I will give my opinion on everything else.

Given your choice in speakers, I wouldn't mind a pair of my own xD I have never heard the amplifier, but if you are happy with it, AWESOME! I would not change it.

I would look at a different approach to your preamp and DAC solution. For instance, since your DAC is for computer audio, I feel having the two separate devices can only improve your sound. I am also not a fan of digital preamps as they sound a little too sterile for my own taste and I have heard them give off noise over the wire when changing the volume. I personally prefer a Class A analog preamp with balanced inputs and outputs for clean and well balanced signals and so I can better integrate pro gear in if I so choose.

I would also take a different look at my DAC as well. I would prefer a DAC that does just one or two things really well, not something that is full of function and features. I have not heard them myself, but the reviews of the Audio Note(http://www.audionote.co.uk) DACs are really promising and they are well priced for what they are. I would also see if they have an option with a USB input, but I personally would get the DAC 4.1 kit (http://www.audionotekits.com/DAC4_1.html) and build it myself. When I go looking for a DAC I always look to see if a USB input in available and located in the back of the unit.

If you're on a budget then I would suggest dumping the money on a great preamp and checking out the Music Hall dac25.3 or the Firestone Audio Fubar II MKII USB DAC. Plug it all into a power conditioner such as an APC H15BLK to start.

I currently own a Topping D2 DAC and I like it a lot for a single ended device. I’m thinking of getting a nice Adcom preamp and power amp to go with it to replace my old analog Yamaha receiver. I am also working on a DIY speaker design using some old Peerless and Scan-Speak drivers in a 2-way tower. My source is an HP laptop running windows 7 with SSD storage (I found that the HDD was making audible noise) using Foobar 2000 to playback FLAC files to the D2 via shielded USB cable from monoprice and a Technics turntable connected to the receiver.

In other words, keep an open mind and explore your options to the fullest.

I hope this was insightful and good luck.