If you cannot afford the Apogee or similar, the MOTU is likely not the weak link in your chain. What are you monitoring through? What are you comparing it to? Is your room treated? Are you listening to raw tracks or mixed and mastered?
I am monitoring through krk rokit 5 monitors, not great. My basement apartment is not treated at all. I compose and produce my tracks and also try to "mix and master" them myself all within Logic and the Mac Pro. Mostly remixes right now. I solved the computer power issue by getting the mac pro, it can handle the track count I couldn't reach on my white macbook. I went for a midrange interface that was known to be stable with macs, the 828mk2. It is stable in terms of latency, no dropouts etc. But I dont know about the sound. Even in itunes, tracks don't sound as clear and direct, the vocals all sound softer for some reason.
As you said, it may not be the motu at this point. I'm obviously lacking in mixing and mastering skills...The main issue I have right now is getting my tracks to sound loud and clear, and I'm not sure buying a 2 channel apogee would solve that or help greatly. I didn't want to buy into the apogee duet hype when you can get way more channels and features on a motu with the same stability. My mixes all sound less than radio loudness and much more muddy right now. Oh yeah, there is a constant buzz/hum sound coming from the motu itself, not the channels, did you get this as well?