Looking to move to an external DAC/amp combo

After using my Titanium HD for about 6 months I'm ready to sell it off and get something external. I can almost always always hear this slight hum/buzz, which I can only attribute to some sort of electrical noise within my case. The other problem is there is no easy switch from headphones to speakers, you have to manually reach around the case and switch the connections. My usage is probably 20% games, 40% music, 40% Blu ray rips played through my computer onto my TV.

Couple questions

1) Do USB devices still use a noticeable amount of CPU?

2) Would an AVR with dolby headphone and pro logic be noticeably better for surround effects than something like the Maverick TubeMagic D1 or D2? Wouldn't an AVR also allow me to connect hdmi out from my video card and have it decode stuff like dolby TrueHD, or is that done by my media player software?

3) I use IEMs almost exclusively, does that affect how preamp vs dedicated amps work with the low impedance? I'm still pretty fuzzy on how amping works..

Budget for this would probably be anything $300 or under. 

I very rarely enable the 3D effect even when playing fps games, so I'm not too concerned about losing that. Frankly I'd rather just be rid of this thing and use the $100 towards something else.

I guess I'm looking at something like the Tubemagic D1, is there anything else in this $200-300 price range that anyone would suggest over that? The amp in that goes up to 600 ohms, does that mean you have to be careful with the volume knob with low impedance IEMs? Is there any risk of damage?

My dad gave me his subwoofer a while ago, which is one reason I'm considering an AVR. It just gives me the option of getting a pair of bookshelf speakers later. How much would you have to spend on an AVR to get sound quality of something like the D1?

bump, to have my question about powerful amps and IEMs answered, and to know what to look for in an AVR. The AVR would only be used for headphones and 2.1, but it needs to have at least optical in and preferably hdmi in. Is there a reason people don't plug there headphones into these that often? Is there noticeable electronic interference?