Agreed - a "high end" DAC isn't going to make a difference if your current DAC already has all the features and decoding that you need. Some "high end" DACs have more features like a volume control or even a remote - I'm not sure how you are controlling the volume of this setup, but if it works for you then stick with the gear that you have.
For future reference, if you ever get a new TV make sure it has analog "Variable Audio Out" - you just connect your powered speakers directly to the TV and use the TV's remote control volume - I like this feature so I always make sure I choose TVs that have it.
I have two different models of M-Audio powered speakers connected to PCs and yes, they are very good speakers especially if you are looking for the simplicity of powered speakers with a 3.5mm input jack on the front - you can't beat them. I agree, if you need a 5.1 DAC and an amplifier then it is easier to just get an AVR which also comes with a remote control volume.
2-Ch (HT L/R): Oppo BDP-105 BD, Adcom GFP-750 pre, Bryston 10B Sub Xover, Bryston 4BSST2 / Paradigm Signature S4 v.2 (L/R), (2) SVS SB12-NSD (Subs)
Home Theater: Bryston 4BSST2 amp / Paradigm CC-590 (C), Outlaw 7700 amp / (4) Def Tech UIW-RSSII (LS/RS/LB/RB), Samsung 46” 3D LCD